Those are the two that I really want, but can't seem to ever find. Although I, like Dacocks, have never come across heroin, I'm pretty sure I could find it if I tried really hard.
2-cb would've been on my list, but there was a spate of ecstasy tablets with 2-cb in them going round these parts last year, so I've tried that.
Mescaline, magic mushrooms, DMT and opium are non existant in their pure chemical form round here as far as I can tell, but all of these are "home-makeable" if you see what I mean.
Other than that, I guess there are the really obscure things that no ordinary dealer has - stuff like AMT or 2-CT-7. But a lot of these are available off the net.
Actually, come to think of it, if I was
really determined to try meth, I could always make some, so the only really impossible one on that list is LSD. I haven't been able to get acid for about three years or more now...