I use the standard 3M sanding blocks, as found in your local hardware or big boxy store, quite a bit:




I know they're not always as flat as they could be, but they get the job done for me, and it's easy to install sandpaper on them.

I acquired the little green 3M sanding block at the left at a recent sale (sorry for the fuzzy picture; don't know what happened), and own a couple of the Warner blocks, to the right, and use them quite a bit too:


You can see the profile of the Warner block, useful for some molding and other tight-spot sanding:


If ever I get around to removing rust from some of my hand saws, I may dedicate one of the Warner blocks to that: nice flat wooden "platen."

I use the  3M type, but also the sanding pads from Harbor Freight.

Quote from: Papaw on February 11, 2020, 04:34:01 PM
I use the  3M type, but also the sanding pads from Harbor Freight.

And I might, too, if I didn't find a sufficient supply of 3M pads at yard/estate sales for 50 cents to a dollar.

I like the size of the green 3M block.  But even though I have a big 3M and the wood Warner type, I always end up using a selection of wood or rubber blocks I've collected over the years.