There is a place here called Princess Auto. They sell mostly farm equipment and accessories as well as trailer parts and welders and other tools and stuff. There is a big section where they sell off surplus items such as used motors of all sorts and other miscellaneous cra ...... er ....stuff.
You should look for a store that sells surplus stuff. Another good motor is one used to turn rotisseries on barbecues. You can get some battery operated ones cheap and might also find one at a second hand shop or yard sale.
By the way. I tried rubber bands with unsatisfactory results. The elastic would stretch and this would cause the final drive wheel to stall when the load caused by the mechanism increased (as in, changed directions from a pull to a push) I would think that there would be less stretch in an O ring so this problem would not arise.
Are you using idler wheels to keep tension on your belts? I have found that using an idler wheel is the best way to keep just the right tension on the drive belt/chain. I am not using bearings in my wheels, but if there is a lot of belt tension I would think a bearing would be necessary for reliable operation. I am using 1 1/2" and 2" bolts so that my wheels turn on the smooth shaft of the bolt instead of on the threads. (sort of a bearing, well, less friction anyway.)
With the chain I am using it is a good thing my motor turns so slow. I don't think it would work well for a high rpm motor. Your O ring belts would be perfect for higher speed orporation. You wold have to build a simple transmission. You can reduce speed quite rapidly going from the smallest pulley on the motor to a much larger pulley with a small pulley glued to the side that would then run a large pulley and so on until you get the rpm you need. (I suppose you already know that)
Anyway, I'm including a pic of the idler wheels on my current mechanism. Also a pic of a cam wheel that will operate stuff in my scene. The cam wheel turns clockwise. There are leavers on the end of the brass arms that you can't see in this pic. To see the thing in action in a short video, go to:
http://s82.photobucket.com/albums/j268/TrainClown/VODEOSTUFF/?action=view¤t=MVI_6645.flv
TC