The Irregular wolves (from their orc wolf riders - the goblins have the same mounts, I think) are a pretty good match, size wise, for the Asgard ones: certainly the best I've seen so far. By that, I mean that the backs are the same height, so that riders of the same size would be sitting at the same level.
The Irregular figures are much cruder than the Asgard ones, but I think they're be a reasonable bet for adding a bit of variety. There seem to be two wolf types: head down and head up (with harness variants too, I think). For me, "head down" is the nicer figure. You can fit Asgard riders onto Irregular wolves without too much trouble.
I would look at Rieder/Alternative Armies too: the Rieder orcs (e.g. "Cro's goblin raiders") work quite well with the Asgard ones (same sort of stature in many cases and some similar hunched body positions), so the wolves and riders might well be quite interchangeable.