What percentage of the booster is built from "off the shelf components"?
A new rocket stage with new tank designs, new engines, and (of course) new landing gear? Not much. You'd probably find some off-the-shelf hardware in the avionics since no one likes to make their own microchips anymore - software's another matter. And I'd bet you'll find a bunch of aerospace-grade fasteners (rivets, bolts, associated hardware like nut plates, etc.) in the Falcon 9 are off-the-shelf. With such a wide range of fastener standards you can usually find anything you want.**
**Usually. 1-inch diameter, 6-inch long bolts made of MP35N are not regularly stocked and vendors lose interest quickly when you only want a dozen, but that's a predicament we backed ourselves into by using a weird bolt.