Before I start I apologize if this is an inappropriate place to post this question; I don't know of a more suitable place. When searching for a proper place to buy fonts I was interested in, a friend suggested that I buy a certain retail product which had those fonts bundled alongwith other fonts making it a good bargain. This seemed like a good idea at first, but I wanted to use the fonts under other applications (The GIMP for images, and OpenOffice, TeX and a non-free PDF library for creating PDFs). I feel I might be making a mistake if I use fonts in this way. I know several people use Adobe's Distiller to make PDFs from Word which include Office fonts, and other Office bundled fonts in other software. Software like StarOffice 5.2 (and the Sun Java JRE distribution) bundle the excellent Lucida family which costs a fair amount of money to purchase. The included documentation just says the font technology is copyrighted, but says nothing about use of these fonts. Is it legal to use these fonts in other applications? If any people from the reader community work at font foundries, I would like to know your opinion. I am sure others have faced this question too.
In general, you can use a font with whatever application you want. Sometimes, however, that may not be allowed. (Iirc, the fonts that come with Acrobat Reader for Linux may not be used elsewhere.)
Creating PDFs is sometimes tricky. Most foundries don't mind if you use their fonts in PDFs (as long as they are subsetted according to their rules). But some do mind and require you purchase an extra license.
So, you pretty much have to check the license. If it doesn't forbid a specific use, you may generally use it.
Personally, I suggest you examine your true needs for fonts. You probably don't need as many as you think you do. I manage quite well in my documents with only two font families (Spectrum and Optima) with Computer Modern filling in math symbols I may need. Most typographers recommend using few fonts, but using quality ones, rather than using as many as you can get ahold of.
Copyright covers derivation, duplication and distribution, not use. Also, in the US typefaces can't be copyrighted, just the programs that implement them. (That's why it's actually legal to use the fonts to print a document, scan it, and re-create outlines of that typeface to make new TTF or PST1 fonts and sell them.)
When creating PDFs it gets tricky, since you're actually embedding the font program, but when creating bitmaps, printed output, or a word processor document with nothing more than references to the fonts you're perfectly ok... unless you've been hit with a shrink-wrap license that restricts you.
In general, the US is pretty far behind most of the rest of the world in typeface protection -- the US copyright office thinks fonts are not artistic works.
The consequence of this is that the design of a typeface can't be protected in the US (but it is protected elsewhere). Howver, the code that implements that face can be patented, and the fonts can also be sold by licence (or license in the US :-) ) so as to limit people's "rights".
The best estimates I've seen indicate that fewer than 5% of commercial fonts are paid for - the rest are stolen.
At any rate, good places to buy fonts include www.myfonts.com (started by John Collins of Bitstream), www.emigre.com, www.linotype.de (you might have to search around for that one), www.fontshop.com and, www.adobe.com/type).
For Linux, you want either TrueType or Type 1 fonts -- Xft and Freetype handle both. Choose PC format rather than Mac format if you're given a choice, because PC format fonts are easier to unpack generally (e.g. with unzip), unless you have a mac around. If you have a choice, get the text face, the italic, and for Type 1, the expert sets for small caps, fractions, ligatures etc. - at some places you can save money by getting just those, and at others you have to buy the bold italic as well, which you'll probably never use unless you do reversed (light on dark) or colour work a lot.
There will be a plethora of files -- a Type 1 will generally have a .inf, a .pfb, a .pfm and a .afm; you need the afm and pfb files, although you should obviously keep them all, as you've paid for them, and the pfm and inf file are used on Windows. Maybe they will be used one day on Linux too. The afm file has font metrics in it, including kerning information. Most Linux software is in the dark ages and uses "single line spacing" and no kerning, but tools like groff and TeX can use the afm file.
I don't know about using TrueType fonts in TeX or groff. Generally Type 1 fonts are higher quality, not because of anything inherent in the file format, but because they are easier to design and hint, so most designers make Type 1 fonts and then convert then to TrueType automatically. The conversion tools have improved a lot, but many type designers still use a 1993 or so version of Fontographer, as the tool wasn't updated. Apache FOP wants Type 1 fonts, or did last time I used it, I think.
There's some OpenType support in some of the Linux tools too, so depending on what you're doing, that may be an option.
If you are looking for a specific font and can't find it, you're welcome to send me email and I'll look in my database or in font catalogues and let you know where to get it, or you could call or email FontShop and ask them.
It can take weeks or months to design, test and hint a high quality font, so the designers do need to get paid. Sometimes they are paid by commission, but in that case the people who paid for font development often want to retain total ownership of the resulting work for hire - e.g. it's a corporate font or for a newspaper or magazine. More often the designer releases fonts hoping to sell them. The going rate for developing Unicode fonts is anywhere from US$10 to over US$100 per glyph, and there are many more glyphs than characters... you can get a good quality custom WGL4 (Western) font designed for $100K or so, if you go to one of the 100 or so top experts. You can get it done more cheaply by employing a student or a homeless person, but the results will often have problems such as certain accent/letter combinations not working or being unreadable for example.