Chris,
I'm not trying to be argumentative when I say the following but it does need to be said. As someone who has been a regular on Colt/Case/Ingersoll discussion forums since 2004 and a Case GT owner since 1978, I do have some experience with belts.
The OEM belts used to power decks, snow blowers and front-mounted PTO's contain Kevlar fiber to give them the necessary strength needed to survive those 90 degree turns that the mule pulleys subject the too. Many, many owners have come on those forums and told us about cheap belts lasting only a few hours max before they rolled over, flipped off or just shredded.
As a fellow Canadian :canada: , I'm a huge fan of Princess Auto but I sincerely doubt that they can source a Kevlar belt and sell if for less than $5.00. Many years ago, I thought that a belt was just a belt and went to Canadian Tire for a replacement deck belt. That one lasted 45 minutes. I thought that I had installed and tensioned it wrong so I went back and bought a second belt. That one did not last 30 minutes.
I bit the bullet, went to my Case dealer and paid the price for the OEM belt. That one lasted for three years and I was cutting a two acre property. Many of these belts are still stocked at most Case/IH dealers or can be ordered in. There are also Power Transmission wholesalers that specialize in bearings, seals, sprockets, chain, belts and other products that carry Kevlar construction belts. Another possible source can be agricultural equipment dealers. Just because they sell Deere, Massey or Kubota does not mean they won't have a belt for your application.
As for belt size, there is more than one size of belt used for every attachment. It depends on the model and year of the tractor as well as the attachment. This is not a "one size fits all " situation. And lastly; belts are a "wear item". They will always fail on you at the worst possible time. That's an unwritten law. Those that understand that law, have a spare belt hanging on a hook in their garage.
I'm not trying to be harsh but those are the realities of owning these tractors. I'm willing to bet that many others will agree with my words as a result of their own similar experiences.