Maybe a little old topic but I just returned to this problem. What to choose for my T-Wolves cavalry? Not interested in joke-conversions, like Marines on Chaos Warhounds, I dug again the net to get something fast and ready. None of these choices below is perfect, but all are acceptable...
1) MrDandy offers a resin set which obviously want to match the look of Canis model. At moderate price you are getting one big advantage - each model is different just from the box, no converting necessary. I strongly advise changing the chinchilla-like tails to the regular wolfy ones.
2) Paulson Games just start offering set of 3 resin models. They are differ in concept from those mentioned above and because total cost (models + shipment) is very similar, it is purely a matter of taste, which set is better.
3) eBay seller irina_est from St. Petersburg (Russia) has two T-wolves at 32 USD apiece. The models shows nice Space Wolves iconografy, not beating about the bush, and each have its own scenic base, but this option is clearly most expensive and still the wolves muzzles are not perfect.
4) Another eBay seller from Russia, mrs-velard has maybe most regular-wolfy proposition, without admixing a monstrous character. The model is the cheapest one from all these, but comes as a single pose. This problem can be partly fixed by cutting the head and repositioning it a little and, by cutting and closing the bottom jaw. Unfortunately, both modifications require re-sculpting the fur at cutting place - small task for some, but big one for the others.
(UPDATE) Now, Paulson Games sells separate wolf heads. Maybe useful to convert this Russian model.
5) Micro Art Studio has two wolf models, now. Very interesting but they do not include saddle nor marine-type legs (but they said at their forum that they are working to offer some in the future).
If still looking for another option, check this post and this one also, please.