28" wheels are 700c wheels. Its not a term commonly used in the States but my understanding is Europeans use it to refer to fat tired 700c treking bikes. In fact the 29er MTBs are also 700c rims just with fat knobbie MTB tires.
As for 700c wheels with disc brakes, its an uncommon combination which mean the availability of inexpensive prebuilt wheels is a little rare. I have been paying $20-25 to build up wheels at my LBS. This includes the price of spokes and nipples. Also check out Ebay and other sites for good used wheels that someone may have upgraded.
Craig