I've also bought a few rolls of adhesive backed sand paper in different grits. This stuff is great because you can stick it to different shape/sized blocks and it's quick and easy to change.
INDASA Adhesive (sticky) Backed Sandpaper Roll - 120 GRIT to suit Dura-Block | eBay
I've also bought a few different sanding blocks but my painter mate Laurie just prefers a flat wooden block about 12" long. He says "The block doesn't lie".
Cheers