The tail panel was used on both the Coronet 500 and the R/T. While they are not as plentiful as the Coronet 440/Super Bee style, the are not particularly "rare" to find.
Source one just like you would any other part: watch parts for sale ads, hit large Mopar swap meets, etc. (I had three to chose from at the 2015 Mn Mopar show and saw several at Carlisle in 2015 & 2016 so they do come up for sale.) As with any other part, price will be equal to condition. Good ones will "not be cheap. Expect to pay several hundred dollars for one in nice shape needing little resto work. Buy the best one you can find.
They are aluminum so they can be easily damaged. Expect to find damage and crease marks on used ones, especially around the key hole. Repair cost will be equal to the amount of damage. Repair cost can exceed the purchase price. Pass on heavily damaged ones and keep looking.
They can be cumbersome to carefully transport and ship. If you find one, make sure you can safely transport it and store it.
Written from experience. Glad I waited to find a good one.