Lowes stocks a large O-ring assortment pack that works well for transmission belts. It is a Danco large o-ring assortment #3, no. 10714. This o-ring set has six different sizes. While one of the O-rings is not usable for me, I actually use the other five with my steam engines and accessories. Oh yeah, the set costs less than $6.00. Take a look, it may be something you can use.
Don Jones