I've also tried desmopressin before and found the same amount of hype, but no worthwhile memory potentiation.
But that's really more of a vasopressin agonist rather than an oxytocin receptor agonist. There are non-peptide oxytocin agonists, check out: Pitt, et al (2004). Non-peptide oxytocin agonists. Bioorg Med Chem Letters. 14: 4585-4589. Obviously, you are never going to find a compound like this commercially available on either normal or grey markets--it would have to be custom synthed.
If you don't mind its pro-oxidant toxic metabolites, MDMA induces the release of oxytocin. Obviously, one should use it as sparingly as possible and take things like acetyl-l-carnitine, alpha-lipoic acid and/or other antioxidants along with it.