It's complicated. :-)

You have to read the EULA. Especially these days few commercial fonts allow modification - official permission must be obtained. The biggest exception is Adobe, bless them. Personally I'm actually OK with violating a "no-mod" EULA clause* but you might not be. In no case however may the modified font be redistributed; the only usual exception is that a third-party (who however must also have a license to the font) may be commissioned to modify a font (something I myself have had the fortune of doing).

* Because I believe it's -generally- unreasonable, sometimes even disrespectful. However, the resultant font must stay firmly confined to the modifier; if it does leak out the modifier must accept full responsibility and face the consequences.

The good news is that these days there are a number of decent "open-source" fonts that are freely modifiable and re-distributable (although very few of those allow you to sell the results, at least not without packaging them in a broader product). I say "decent" however because -as one might expect with free products- such fonts often don't perform adequately when push comes to shove; the amount of time fixing things one shouldn't have to fix can offset the cost of modifying a commercial font... There are exceptions, but it's hard to identify them without serious research. We actually have an active forum member by the name of Charles who has a good deal of experience using open-source fonts in professional projects - hopefully he'll help you find a suitable one.

BTW, I assume you have some experience using a font editor? It's no cakewalk, although very basic modifications can proceed pretty smoothly.

hhp

Or...

Free IPA fonts
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=FontDownload...
http://www.phon.ucl.ac.uk/home/wells/fonts.htm
http://www.wazu.jp/gallery/Fonts_IPA.html

If you need redistribution rights, you either need a lot of money or use an OpenSource font.

Nick wrote:

http://www.phon.ucl.ac.uk/home/wells/fonts.htm

I wouldn’t recommend these, they aren’t Unicode-encoded, which is a no-no if you want interoperable text.

Most (free) phonetic fonts seem to look like hack jobs by linguists with no type design skills whatsoever, so you have to be really careful if you want to use an existing font.

The DejaVu fonts look decent in my opinion.

Gentium is OK but it’s a very specific style that doesn’t necessarily fit with your project.

Charis SIL is a good, robust workhorse font.

Thank you for your responses.

So, I don't have a problem when doing this with an Open Source Font, right?

Do you have any recommendations for good Open Source Fonts? Is there a good list out there?

What do you mean by "you need a lot of money"? To pay the designer of the original font to use the font?

Let's put "redistribution" aside for one moment. Would it be possible to just buy a big family and alter the one version I need for my project. Do you think it is possible to speak directly to the designer of the font and ask him if I'm allowed to do that?

Do you have any recommendations for good Open Source Fonts? Is there a good list out there?

I don't know about good but certainly quite exhaustive...

http://typophile.com/node/97575

Reynir went to considerable lengths to compile an overview of what is available from different sources.

The Adobe Source Sans Pro fonts, released last year come with the level of polish and professional development that you would expect from Adobe's type team (specifically, Paul Hunt) and as such there are few better recommendations. It made Typographica's Favorite Typefaces of 2012 list along with another typeface, Alegraya variations of which are available under an open source license.

Would it be possible to just buy a big family and alter the one version I need for my project. Do you think it is possible to speak directly to the designer of the font and ask him if I'm allowed to do that?

You should be able to do this with Adobe's original fonts (those for which the IP resides with them) subject to the license agreement terms and conditions (which include stipulations around how and where the modified fonts can be used). Contacting the copyright holder in such matters is always advisable as they will be able to give you a definitive answer on what you can and cannot do with the fonts.

The only restriction on most open-source (AKA "libre") fonts is that you can't sell them (or modified versions) by themselves (although you can sell a broader packaged product).

It depends on your standards, but there aren't many really good ones, as attested by the fact that out of Typographica's 55 "favorite" fonts of 2012 (that Karl linked to) only at most 1.5 are free.

If you ask the designer of a non-libre (AKA "proprietary") font for permission to modify it, he will probably demand a fee (or just say no). And the only way to redistribute such a font is to buy the rights, which typically costs thousands of dollars.

I think your best bet is to see if there's a free (not necessarily libre) font that suits your needs and is of sufficient quality. Failing that you should be able to find a libre font that comes close, and pay somebody* to extend -and often correct- things for you, or do it yourself. Depending on how much work that libre font would need, it might be cheaper to go with an Adobe font instead.

* Like yours truly. :-)
hpapazian at gmail dot com
There are however many others reading this that can do the job for you.

hhp

What do you mean by "you need a lot of money"? To pay the designer of the original font to use the font?

Not to use the font, but to get the rights from the original designer or foundry to be allowed to redistribute the fonts.

Do you have any recommendations for good Open Source Fonts? Is there a good list out there?

I listed 3 Open Source fonts which already have phonetic characters.