89 t-bird fast idle

89 Ford T-bird, 3.8L engine, no supercharger. Suddenly developed fast idle problem. Dealer claims I need a new throttle body.
Drove home 300 miles from vacation on Sunday. Car ran great. Turned car off to drop off passanger. Started ok. Turned car off to unload my luggage. Car started ok. Pulled into a parking space, and when I put the car in park, suddenly began fast idle. Approx 2,000 RPM in park. Approx 1,100 in drive.
Brought it to the dealer today. They claim I need a new throttle body. They said diagnostics checked out ok. I'm sure that dealer did not remove throttle body, external inspection only. Says internal plate stuck. Problem is that throttle body is no longer manufactured. Need to find used or rebuilt.
Also need muffler - baffles shot. Dealer and Meineke both say that obstructed muffler would not cause fast idle problem. Meineke won't replace muffler until idle corrected.
Does this sound reasonable? Any suggestions? Any recommendations for used parts dealer on line?
Thanks

When my SC had a high idle I had to change the Mass
Air Flow sensor.Also idle speed control valve.I believe
the standard bird also has these parts.

Thanks for the reply. Brought it to an independent garage today, and they replaced the idle air control motor. Runs great again. Thanks. Chris