I bought a 26 x 1 3/8 coaster rear only recently. I think it was from Taylors wheels who although sounding uqite British, are actually German. Worked out cheaper for me to buy a complete wheel than to buy a rim and spokes and build a new wheel with the hub from my old wheel.
There's a seller on eBay does these.
Think they do singlespeed freewheel and coaster.
Bout £60 a set last I looked I think.