Can't find a rear replacement wheel 29" x 2.125 coaster brake - single speed
If you're in the US then almost any bike shop can easily order you this wheel from J&amp;B Importers, or this one. Those have Weinmann 519 rims, which are kind of more hybrid than cruiser/mountain - around 24mm wide, whereas a proper balloon tire cruiser rim is around 30-32mm. But if you want to go narrower than what's probably on there, it will be fine. 

A bunch of dealer-level brands have made 29er cruisers, Torker and Felt in the US market that I can think of offhand, so one way of procuring an inexpensive pre-built 29er coaster cruiser wheel can be from them through their dealers. Usually that kind of thing will mean a call from the dealer to the company to see if they have any as replacement parts, as opposed to being a normal catalog item. Torker is a mostly non-territorial  brand of Accell North America that most shops with an account with them (which, again if you're in the US, is most of them) can get, so if a Torker T29 (the name of their fairly long-running 29er coaster brake cruiser) rear replacement wheel is available, any shop you want to work with can probably get it.

If nothing in that direction pans out, or if you want to spend a little more to get higher quality, then just building such a wheel is something any good shop can do.