mrs-velard's thunderwolf: a friend of mine found it on ebay, it's a decent model but I think it looks a bit silly and doesn't look like the thunderwolves on the codex
Max Mini's Giant wolf: it comes with a removable back, it even has some different heads
Tharn wolf riders: designed for the game Hordes they are a little bit too small for a space marine and again they don't look like the thunderwolves on the codex
update:
Micro Art Studio has a couple of really nice wolf models that you can either use as Fenrisian wolves or as Thunderwolf rider
