fludan.com has DMSA. They are a Canadian company, so if you're in the U.S. you can buy up to a 90 day supply without a scriipt legally. There are some pretty large quantities on there, so I would email the company and ask what amount you can order - wouldn't want it stuck in customs.
I buy nystatin from this company and the quality and price is excellent. It does take 3 or 4 weeks to get my order, though.