The arrow is the easy part!
Arrow - 5/16th driveway marker. 32" x 5/16th is standard, shorten it to stiffen it.
Nock - factory plastic or self nock in the end.
Tip - Could convert any old tip or even a field point to a good holding head with piano wire and make a swing way style. It really could be anything as long as the end is sharp and prevents the fiberglass from splintering. You can latch the piano wire right into the glass arrow - I have not had splintering problems with that.
Safety slide - use an empty Rem 20 ga hull or a Win AA. A vise squeeze will take care of the steel head. Drill out the primer pocket with a 3/8th or maybe a hair smaller to fit around the arrow. I forget what size I used to use. Cut the hull down to about 5/8". Drill pass holes through the meat of the basewad.
Slide stop - A piece of rubber under an oval head screw...or just the screw head. The rubber is just a buffer.
I used the slides for years before I bought any for $3 per arrow and they work fine. They actually slide better than the AMS slides do.
Good luck with the bow, sounds like a good project. Bowstring and bowfishing line would be the parts I would buy outright, though you could easily learn to tie a bowstring as well from proper material.
EDIT: I say a rem 20 ga or win AA because they both have integral basewads as opposed to separate paper/fiber/plastic basewads. And 20 ga because that is big enough and smaller than 12 ga.