The mesc juice? Hmmmm, I cannot help you there. I can tell you that you need to make a tea, and if you are going for pure mescaline, it won't be "juice". You could just let the cactus dry in a cool, dark place and eat it dry.
Google is you friend. I'd recommend buying dried skin of the cactus of your choice (not the powder, it tends to lose potency much faster) and then making a tea from it. Eating it can cause severe nausea for some people.
this is a shameless plug but.. my mister sells the powder/skins on ebay.. http://cgi.ebay.com/1LB-Peruvian-Torch-Incense-Powder-San-PedroRelative_W0QQitemZ170276725203QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item170276725203&_trksid=p3286.c0.m14&_trkparms=72%3A1205|66%3A2|65%3A12|39%3A1|240%3A1318
iggy: for shame. there are vendors who will do half a kilo for that price. and whole skins too so you can see the amount of green material and white.