Foamez sells them.
I made mine out of balsa. Nice and light. Another handy sanding block is nothing but one of those wood paint stir sticks. I have one with 40grit. Another with 80 grit. Just use contact cement to glue the paper. I have a wood shop and have built tons of sanding blocks but I swear the paint sticks are the best. Work on cleaning up laps on glass jobs also…
It would have been a lot easier with Balsa! I just glued up some Poplar I had laying around. Man I miss my joiner when i do stuff like this. Pickin up some balsa this week for a concave block. I will make up a paint stick block as well, thanks.