Get Zicam cough spray. Easily the best source of DXM, which is a dissociative. Each regular bottle contains something like 310mg of dex, and each bottle of Z-Nite has 500ish-mg of dex which I wouldn't reccomend for the first time trying the drug.

No other OTC is really worth while. You could always go to a local nursery and get some Morning Glory or Hawiian Baby Rosewood seeds - they contain LSA which is another psychadelic. You'd have to go through an extraction process or at least wash off the seeds before hand, however. I'd reccomend an extraction, though.

www.erowid.org - look it up.