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I just figured this out so I will answer my own question to possibly help someone in the future. I bought this replacement fob from Amazon: https://www.amazon.com/gp/product/B00KTJ807W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It replaces the original fob from my 2005 Park Avenue which I lost the part number for. Using an Autel AP200 bluetooth OBD2 scanner, go into the Immobilizer Service option, and it gives the option to reprogram fobs. Start the process, hold down the lock and unlock buttons together, and it is done in 15-30 seconds.
The local Buick dealer was going to charge "$75-$100" to program the fob and had no guarantee it would work. So the scan tool paid for itself and now I have a scan tool . Unfortunatley not every OBD2 scanner will let you do fob programming. You need to look for one that does for this to work