On the blackmarket, you can buy one gram of MDMA for 20-25 USD, shipping included. Price goes down if you order larger amounts. Here's how you can do it:
1) Download and install "Tor browser bundle"
2) Buy some Bitcoins (BTC). Virvox is convenient and allows you to use a CC.
3) Run Tor Browser and register to Agora Market using this link:
http://agorahooawayyfoe.onion/register/i3PmGLEeJx
4) Send your bitcoins in Virvox to the address in your Agora profile.
5) Learn to use GPG. Gpg4win is a nice bundle. You don't necessarily need it but it is advised that you encrypt sensitive info such as your address before sending it to a vendor.
6) Buy whatever you like. The market is pretty much like Ebay. Buy from reputable vendors. Search the forums if in doubt.