I was wondering how I would go about installing a set in neck. And I was also wondering if it is possible to do it with a neck from warmoth (a bolt on) or if I would have to make one from scratch (which in that case I would just install a bolt on neck to save myself some trouble).
i don't think that a bolt on neck would have a long enough tenon for doing as a set neck. you'd have to put in screws as well.
You could glue it, but most bolt on necks aren't made to the same tight tolerances that set necks are. The PRS neck joint doesn't really have much of a tennon, but it holds on okay...maybe you can find a happy medium between the two.
Is there maybe a website that sells set-in necks?
Probably not. There are bolt-on necks with more comfortable neck joints.