Contact Dick at North race Cars. He has some beutifull Team III wheels. Although not listed on the web site, I think I remember him saying once that he did have 4 lug wheels available.
http://www.northracecars.com/wheelofferings.html
Also contact Vintage Wheels. They advertise on the other board, but I don't know anything about them.
http://www.vintagewheelsus.com/arWheels.php
For seats, finding stock low back seats is pretty easy. They're available from a few venders. And you see them for sale on the boards pretty often. And, there's a reason for that; low back seats have a definate safety concern. There's a reason you don't see low back seats in cars built in the last 25 years or so. If you do buy a seat of low backs, you culd easily recoup your money selling the racing seats you have now.