Jim,
There are many sources of covers, and many materials as well. Expect to pay USD $150-$250 depending on your needs. Just poke around the internet and you'll find a dozen vendors.
For inside storage only, and dust protection only, cotton flannel is the least cost alternative.
For outside storage, or protection from sun and moisture, there are various non-woven fabrics and kinds. It really depends on what you need.
A welcome tool to have is the California duster, a hand dust mop whose head is impregnated with some kind of solvents to pick up dust. They work well to keep a clean car clean, inbetween washings. Just sitting anywhere, the car will attract dust.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored