You can buy belt sanders from a local store like Sears and True Value Hardware.
Take the time to learn and do it right the first time - you may not get a second chance.
Some of the supplies I list below are optional - you don't have to have them all.
This list will also provide hints about what supplies will make the task of tree-ring dating even easier.
The fine art of sanding wood is discussed in a vast amount of trade journals.
Even your local wood shop can tell you how best to sand your specimens, but keep in mind that surface is everything to a dendrochronologist!