I really go for DKNY cotton stretchy thongs. I've found them to be even more comfortable than the VS cotton thongs. You can find the DKNY ones at any upscale retailer like Nordstrom's, and invariably they're on sale...I've never seen them not on some kind of sale
One recommendation I would make, though, is that your wife put on a thong and wear it in a comfortable situation all day--like lazing around the house. It will get her used to wearing one and make it easier to wear in an everyday kind of situation in place of regular underwear. But for some people wearing a thong is just not something that comes easy or naturally.