Here's what I found, toss out your power sander. I spent well over $100.00 on paper for my brand new random orbit sander. Only to find on my NE Dory that a hand sander with 80 grit paper worked better and faster. It can get into all the corners, which your power sander cannot. The only drawback is the lack of dust collection so a good respirator is a must. For the painted surfaces you don't need anything finer that 120 grit, for the clear coats 220 seemed more than adequate. If you want to use your power tools, I found a place on line that had good prices and good quality. I think it was called Online Industrial Supply. Check them out, compare prices, have fun!

I like Klingspor abrasives.  You can find them online or at woodworking stores. They have always seemed to work better for me.

That said I forgot to order and ran out of discs on my new boat so I ran into the big orange store on a Sunday afternoon and picked up some sanding discs from Diablo. They seemed to work just fine, price was reasonable and they had a big selection of grits.