Build your own with a pillow block. This way you can have different grits on each side. Save changing back and forth. Also, you can change out the discs for muslin pads if you wish to polish. You will also be able to run different grits on each side with the pads.
The backers for the stick on sanding discs can be cut from masonite or aluminum. I use masonite as it is cheap, plentiful, and easy to work with.
If you are good at scrounging and making stuff, it is cheaper too.
Not sure of what you mean with the pillow block. I am good at scrounging though and a friend is delivering a washing machine motor later today. Not sure if it will be at the correct rpm though....
I have a "how to install recoil pad" tape and one of the guys has nice sander that looks like it's on a grinder motor and mounted on some 3/4" plywood and I have a few pieces of that.