My rear wheel's rim is severely damaged and I want to replace the rim. I don't want to respoke it, so first question, is it possible to buy new prebuilt wheel somehow without the transmission itself, may be with hub shell, without hub internals? Or I have to buy rim and respoke it?
I don't think you're going to be able to swap the guts of an internally geared hub on to a prebuilt wheel that doesn't have the same hub on it to start.
If you buy a new rim, you should be able to do a rim swap relatively easy if you have a truing stand. Simply tape the rims together and swap the spokes over to the new rim one at a time. Then you'll need to re-tension the wheel from scratch, but you won't have to worry about re-lacing the wheel. If you're not comfortable with this process, you may want to take it to a bike shop and have them rebuild the wheel for you.
Here's an overview on how to do a rim swap in more detail.