I may be hopeful, but I say yes, it is possible to save an Oncidium with one green pbulb and some healthy roots - IF it has any living eyes on it. You can find the eyes at the base of the pseudobulb, under the first "sheathing" set of leaves. This pair of leaves are the first to appear on a new pbulb, and will be the lowest ones on the pbulb, flanking either side of it, and coming from the base. If there's any healthy eyes - even one - there's a chance it'll come back.
As for where to order them, I honestly have no clue. Of the few growers I still know of, none of them have many, if any Oncidiums or relatives at all.
Lastly, a bit of advice/personal experience: I've never done very well with Oncidiums and their relatives. I've always grown indoors, under artificial light. Either I don't keep them humid enough, or I don't keep their potting mix moist enough. The only Oncidium I've ever had success with was Oncid. Sharry Baby - one of the better known plants back when I first started growing, with LOTS of flowers that smell like a mix of chocolate and vanilla. If you're looking to get into Oncid's, and grow indoors, I'd recommend reading up on semi-hydro on
FirstRay's as an option to keeping them happy. It's the only way of growing that's worked for me and my Oncid's.
Hopefully this helps. Happy growing!
-Tim