You can pickup a S-A 3speed coaster brake hub for pretty reasonable, but I would not get the TCS version, they are more of a slowing device than an actual brake

(don't ask me how I know that one!) There are also Shimano 3speed coaster brake hubs out there too. I am a cheap rascal and would be searching the thrift shops, dumpsters, scrap yards, bike shop bargain piles, etc for a donor wheel, rather than plunking down the $$ for a new hub.
Aaron