There are a couple options I can think of. Skinz (
http://www.skinzwear.com) has a couple G-string designs. They're a little on the expensive side, and you only get a couple color choices unless you plunk down another US$10.00 for a "custom order." It's not technically a G-string, but you can probably achieve the same effect with a 3/8" side suit from Muscleskins (
http://www.muscleskins.com) with a thong back. Given a choice between the two, I prefer muscleskins, because the price is a bit lower, and you can get it custom made to order, within reason, for the same base price.