I am looking at building something similar. Here is what I have found on tracks (note I have not filtered this on cost).
http://www.mcmaster.com/#timing-belts/=klc1d2 for timing belts
McMaster-Carr for conveyor belts
I found another provider of conveyor belts of the right size for what you are after but can't lay my hands on the URL at this time
The Chinese produce a number of specialist tracks
You might be lucky and find an old track snowblower on ebay
Tell me how you get on
Cheers
Bruce