I would think that most of the Argonauts would be represented just fine by any of the hoplite ranges out there. A box of Victrix plastics would cover pretty much the entire crew, other than a few distinct characters like Herakles, Zetes & brother, and maybe Orpheus (if you really want him to have that lyre).
Off the top of my head, I can only think of the Reaper and Northstar models for Herakles. Not sure if either is cheaper than Foundry but they are sold individually, so might be more convenient, in any case.
I don't know of anyone besides Foundry that makes Zetes, and even then they don't make his brother, but attaching a pair of wings to the back of an otherwise ordinary model doesn't sound too hard.
For Orpheus, you could always just paint a lyre onto his shield to mark him out.
I'm afraid to say that I don't know of any six-armed men, and I know you stated you don't do conversions, but using plastics of some kind would make it exceptionally easy to pull this off, if you wanted to. I would do one pair of arms below the other, and the third pair behind and between those.