Apparently you can get it if you ask the staff - I think it's sold on naturopath's advice only. So try one of the bigger health shops. ALA is very short acting, so unless you get a sustained release formula, I believe you need to re-dose frequently.
Failing that, I am rather fond of another antioxidant called grape seed extract. Although there are no studies on using it to reduce MDMA related neurotoxicity, logica suggests it should be helpful. It is very long acting compared to vitamin c and ALA, which is a bonus. Apparently it can also reactivate vitamin c and e when they have been used up, prolonging their effect. It's not cheap but it is easy to obtain.