Not online but I bought a bunch of it from harbor freight. Wet sand from Eastman, the discs. Also was a small paint and auto body supply store that stocks stuff like that.Stuff like bondo, paint, spray guns, and what not.
I buy quite a bit from Autobodytoolmart.com. $99 and over get free shipping so I usually get a list together. I usually stick with the 3m red abrasives and 3m wet paper. last go around I bought a roll of their house brand Champ. It was ok. On the color sanding paper, there is a metric grit scale some of the imported papers carry that is not exactly the same as 3m paper ie; the 1200 cuts more course. The thing about sandpaper you'll use twice the amount of cheap paper as you would 3m gold. The 3m red is somewhere in between price and performance. The Indasa, Mirka, and Norton paper I have used I didn't like. Sounded like I'm plugging them , but I have been using them for years and never an issue with customer service.
I agree with paper. buy the best brand, same with tape. use the good color, if it will stay on a while! like said, get free shipping, the good 3 m brand, give it hack! auto paint supplies are cheap, but a roll will last for several cars.
I get all my body stuff, sandpaper, putty, sanding blocks ect. from AMAZON I use all 3m sandpaper & in most cases it is 1/2 the price of the local store