Answer:
"By a prescription from a physican"
disregard what that imbecile wrote. you can find all sorts of extracts, and seeds on many many sites. these are useless to you. belladonna is a pain to germinate, and takes quite some time to grow. the extract will not induce the "trip" that you are going for. it is also not illegal.
http://www.alibaba.com/showroom/atropa-belladonna-leaf.html