Can anyone tell me if a 338-378 will work in a Sako action -- if so, will it work as a repeater? Also are there any sources out there for just the actions? Thanks

Yes it will in fact Sako sells a model 995 in 30-378,338-378 and the 338 Lapua Mag which can also be had in the TRG 42.

I agree that it will work in the 995 (TRG-s) with its detachable magazine, but I'm not sure how difficult it would be to expand the magazine of one of the L61R or AV models, which have a magazine of just about 3.6". I think that anything on the .378 WBY case is going to need at least 3.75" or more. Just a thought.

Is it possible to buy just the 995 action?

You can look around but, Sako will not sell just an action. I tried.