Just get em from Mcmaster-Carr
You can futz with the size but I use 8-32 thread as thats least invasive in terms of drill size, and you can still clamp necks on comically tight.
I would not buy zinc ones as they just don't hold up. Brass is fine if you're careful not to cross thread etc. If you use a drill to attach neck screws at all I'd use steel. I use a drill and brass and I've never had a problem but while I may be stupid, I'm not an idiot. I like stainless over non stainless but the price difference is like 5x.
Zinc plated Steel (not zinc, zinc-plated STEEL).https://www.mcmaster.com/90192A157
Yeah maple will actually hold machine threads well too, especially cross-grain like that, but you do have to tap it if you want it to hold multiple in/outs in my experience, and I'd use a larger thread. I prefer inserts because I don't see a benefit to not having them, and they're not that expensive; even if you say $20 for 4 for stainless with excessive shipping isn't much in the scale of how much a guitar often costs