Most uppers that are designed for Glock mags should work. I'd buy the components separately and then assemble yourself. You just need a way to clamp the upper while tightening the barrel nut, I'd be happy to share the clamp prints I use (you need a vise to use them).
You want a barrel with a chamfered breach, as they appear to have less feeding issues in general. Though I've not had reports of any feeding issues regardless of barrel (yet).
The KAK bolts are what I use.
