Welcome,
Nice to hear of someone bringing another CB750 back to life, I'm currently rebuilding my 1976F1, it has sat in my garage for 12 years while I was working overseas, it's a bit heartbreaking as it was roadworthy when I left in 1994.
I get parts either from CMS, David Silver spares, or from eBay, just got a load of F1 bits in a job lot from eBay, also this weekend I'm taking delivery of a top end from an F2 (I've got one as well).
Engines do come up on eBay, recently there was one from a K series with 12k on the clock, I cannot remember what it eventually went for but it started out as