Recreation Supply in Saskatoon keeps Whal extroverts in stock for this application, wholesale only mind you. Go to any Dealer in town and get them through them, if issues, call me and I will order them and then ship them back to you in Saskatoon, ..hahaha.
I run avids on my 08 mtx for the powerclaw track. Robbie's get to deal with. I have read if you are running boost or planning on running boost. They are the only one to go with. The other work well for n/a sleds. Avids just seem to last longer and handle boost better