I thought of a couple of things that will give you lots of crazy dreams (not sure if that's what you are after), don't really know about REM sleep per se...
5-meo-amt (which is not impossible to get), will give you lots of dreams for several nights after you take some, and they usually are extremely bizarre and generally pleasant (but occasionally very disturbing...).
You can also nibble a bit of datura leaf (available online or in your neighbor's front yard), if you wanna take a walk on the wild side. This tends to produce dreams that are rather dark or even nightmarish. Once again, the effect will generally last for a few days. Of course, you have to be somewhat careful b/c datura can be quite bad for you if you consume too much.
Please note, this is all based solely on my experience, and thus should be taken with a grain of salt!
~Maitland