Barbiturates aren't really that easy to find anymore since Benzodiazepines have basically replaced their function without the high risk of death, though I do see Soma (Carisoprodol) which while not a Barbiturate itself, has very similar properties including the high chance of a fatal overdose (if my memory serves me correct).
If you're looking for a website to buy that stuff just check tor.taxi for official links for those markets. I think Archetype is the biggest at the moment.