Check out chessmate.com http://www.chessmate.com/chessmate1.html
If you scroll near the bottom of that page you'll find kits of those type pieces. I don't know if they'll sell individual pieces but it couldn't hurt to ask.
I don't have any of their products but they look like some of the nicer travel sets I've seen.
I lost a piece from one of those "checkbook" chess sets and just wondering if anyone knows where to purchase some