Another angle .... you can drive over to your local NAPA store and still buy some to a lot of what you need there. Ignition parts aren't much of a problem there. They have fuel pumps and water pumps. They have tie rod ends and bearings. Go to RockAuto (
http://www.rockauto.com) and check them out. They have quite a few parts online for '49 plymouths. Hubcaps ..... some aftermarkets (moonies) work just fine or you can find original style caps on eBay. Interior parts ..... cloth a LeBarron-Bonney and SMS. Clips, windlace, headliner, and door panel material at Restoration Specialties. Wiring at Rhode Island Wire. Plus, keep in mind that many, if not most of the larger parts within your car can be rebuilt including small electric motors, entire engines, heater cores, starters, generators, fuel pumps, distributors etc etc.
And, on your seat covers ..... do you want to restore the seat or just cover the seats. Restorations .... go to LeBarron-Bonney or SMS or ??? others exist. But, to just cover them. Look on eBay for slip covers. Check out JC Whitney for headliner material. Go to JoAnn's Fabrics for basic auto-style fabric that will work and doesn't cost an arm and a leg. Go to a local autobody place to see if you can get some door panel core to build new platforms for your doors.
John
Colorado