I doubt you'd be able to get DMSA on prescription. You can buy both DMSA and ALA from http://www.nutricentre.com. DMSA is sold as "Captomer" by Thorne Research. I got the Kirkmans brand of ALA.
You can also get DMSA from http://www.supersmart.com, though it took about 3 weeks to arrive. It is cheaper though. Don't know how it compares as I used Captomer for my first round.