There are a number of places to buy Case parts. I have used http://www.colemanequip.com/Case_Parts.asp. Most of the places that sell parts have fairly nice exploded diagrams of the parts. You can get the specs for the parts, including the belts, from that site. The standard 1816 had three belts. There isn't much adjustment travel, so you might have to go a size smaller or larger. Also, over the years these things have been modified with different engines and mounting set ups. On the standard set up, the motor had to be at least loosened to get the belts on the drive pulley. It's not clear from your post whether you have the motor, and it was not in place, or whether there was no motor at all with your machine. If you have the old Tecumseh, just slide it in and see if the belt fits. All three are I think the same size. They are standard V belts you can get at any parts store.