I have a 3x21 inch craftsman belt sander. It's about 12 years old. With the sand paper belt is mounted and tightened and I start to sand something the belt will slip and not sand. I checked the tightening latch and it seems OK. Also the rubber drive roller is loose and comes off easily, Should this roller do this? I can't get anything done in this condition.

try tightening belt buy using adjustments. refer to instructions.

The belt you can replace with a is simply any generic 3x21 belt. You can find them at your local hardware store or order them from a website like Home Depot, etc and have them shipped to your home. Here is a link to take you to Home Depot specifically for an 80 grit, but you can look for other grits as well. 3" is your width and 21" is the diameter of the belt you need.
Gyros 3 in 21 in 80 Grit Aluminum Oxide Sanding Belt 5 Pack 12 32180 5 The...

My Porter Cable 352vs Belt Sander was clogged from sanding epoxy, wet wood, something like that. No dust collection at all.
I had to remove 4 screws on the housing, one screw under the front handle and another below the rear handle. The housing came out very easily (with a little wiggle). Pay very close attention to where the position of your plastic fan housing sits. It will only be installed in one spot.
http://www.ereplacementparts.com/images/porter_cable/352_TYPE_1.pdf
The pdf diagram was not exactly correct on the housing placement.
Once opened, The wood and epoxy were caked in everyplace. I brushed them out and replaced the screws.

Good as new.

Good luck.

is it a B & D 3x21 belt saNDER. iF SO, PUST THE REAR DRIVE WHEEL FORWARD UNTIL IT CATCHES, PUT THE SANDING BELT ON, THEN RELEASE THE CATCH WITH A LONG SCREWDRIVER ANDIT WILL SNAP BACK INTO PLACE. hOPE THIS HELPS

There is a tracking adjustment knob on the side of the sander--adjust one way or the other until it tracks properly. This is something that will change by itself as the sander is being used--check periodically.

There is a leveling (screw/knob) on the side of the belt sander. Turn it up and the belt will drift in one direction. Turn it downward and the belt will drift in the opposite direction. play with this leveling screw/knob until you reach a good balance and the belt will stop drifting.

All sander will have two adjustments. The first is to release the sanding belt by retracting the position of one of the rollers. The second adjustment moves one of the rollers belt tension from side to side. The roller angular position is changed, sometime by a single knob or screw, and sometimes by two separate adjusting screws.

Do you know anyone with a metal lathe or that works in a machine shop? If you do ask them if they can make the rollers for you out of metal. Or else find a machine shop and ask them to make you new rollers.

Motor may just need new brushes, Cooling vents may be clogged with dust. or bearings in the motor or front roller maybe bad. Causing excessive drag
blow it all out with compressed air or shopvac
look for external brush caps (2) oppsite from each other take them out to be sure they slide freely, sometimes one gets hug up and doesn't make good contact

try to rig the nozzle of a shop vac to the part where the bag would normally connect and run the sander simultaneously with vac for a few minutes and see if that helps