you didn't mention button shapes or any "special effects" you might want but i found
this site via a quick search ("button maker"). some of these online button generators are pretty good and this one is quite inclusive with many customization options, including uploading your own fonts, textures, etc, if you wish.
note, non-members of the site are limited to buttons 100 pixels wide and i don't know what art for iphones is about spec-wise but, since you're willing to pay some (site charges $9/mo subscription), if 100x100 isn't big enough, you could go that route.
also, after you follow the creation steps (make sure you take note of the color values, fonts, etc so you can maintain uniformity. there are a number of options here) and then hit the "make the button" button on the bottom, page 2 will preview your button toward the top and then below it will have 2 links to the separate "normal" and "selected" graphics. the file types available are adequate, too (chosen on first page, including .png).
anyway. i think it's what you're looking for. good luck
example added: