Ebay item: 274672064928 (there are several sellers so shop around who has the best price & feedback and also ships from the US)
Coupler: 8mm to 8mm Ebay item: 152161784190...(again, shop around, the prices vary on these. I only paid a few bucks for mine)
FYI....The newer model Rollsizers now rotate counter clockwise. The speed control that comes with the motor can be wired to accomodate this. Clockwise or counter clockwise. According to Kevin from Rollsizer.com, the reason they did this, is because folks that have the earlier versions that rotate clockwise, were using too powerful of a motor or drill and breaking the bolt that turns the shaft. With the shaft on the newer models that now turn counter clockwise, if there is a jam, the bolt will break loose the loctite and loosen rather than actually shearing off the bolt.
You'll want to get the type of coupler that "flexes" in case there is any slight misalignment between shafts.
Disclaimer: Any of the above mods are "at your own risk".