A widely used reagent is Marquis and I would advice you to buy that ready as it is since it involves some nasty chemicals.
This is a nice site for that:
http://bunkpolice.org/Though do know a VERY important thing about reagents:
One reagent can tell you if your substance is NOT what you think it is, but it can NEVER tell you for sure what your substance actually IS.
There are several chemicals that react the same way to a reagent and will make the same coloring, so if you have a certain coloring there will be a wide range of chemicals to choose from that is causing that coloration.
To reiterate: One reagent can show you that your MDMA is actually no MDMA at all, but it can never tell you for sure that your MDMA is really MDMA, it can only hint at it and further tests are advised.
Kind regards,
The Traveler