When I sanded out my NACRA Sq I used an electric sander, so it really wasn't that hard work
I began with a pretty aggressive grit paper. It might have been in the 60 range, but for some reason I keep thinking it was 85
I sanded, all over the boat, front to back, then went back to front again. When I just began to see black or bits of a different colour beneath the grey (the old NACRA had grey gelcoat) I called that 'done.'
I didn't sand every last bit of gelcoat off, and since I was painting over, I am pretty sure my finish sanding was in the 80-grit range to give 'tooth' to the surface for good paint adhesion.
Once painted, THAT was when the work commenced... wet-sanding, by hand, to a 1200 grit finish
As far as places to buy... well, unless you just want to deal with purchasing a rig, you can take the COMPLETELY PREPPED!!! sanded, taped off whatever ou want to block, that sort of thing... hulls to a commercial paint house and get them to hit your boat.