You can go to cafepress.com and upload your own images - they'll silkscreen them onto a thong or bikini briefs. Unfortunately they only have white ones, but you can upload whatever graphic you want. I have a thong that says "bite me" in elvish
Edit: I've never used these guys, but they turned up in a google search for personalized underwear:
<a href="http://www.debracraft.com/">
http://www.debracraft.com/</a>