Tim,
Some thoughts occur:
You can also attempt to measure your belt size by running some twine around the pulleys and marking the length, some careful measurement will get the profile. Draw up the cross-section of the pulley profile and/or the belt.
1. Are your belts V-belts? A lot of older, established hardware stores, farm implement dealers, and auto-parts stores have devices to measure existing belts, then consult their source information to find one closer to the size you need. Some suppliers even have cross-referencing data available (Yes, I found the belt I needed for a 30-year-old off-brand washing machine in a list with a current product part number)
OR
2. Once you have measured the distance around the pulleys, consult sources like McMaster-Carr or Grainger to see if they have something close.
OR
3. That looks like a mill also sold by other vendors (Grizzly?) Perhaps getting actual belt sizing information from one of those sources might help you narrow things down.
OR
4. Maybe one of the link-type belts can replace a V-belt, or someone sells a kit to make a custom belt (stranger things have happened.)
OR
5. Best approach is if someone has that mill and knows what the belts are, as has been mentioned. (I don't know for sure if this helps, but the "-R8" suggests that there are similar mills with other than a R8 spindle.)
Hope this spurs your creative juices.
--ShopShoe
