Congrats on the purchase -those Park Ave Ultra'* are some of the smoothest riding cars ever from any of the 'Big 3'.
Your best bet for the tensioner is to check your local salvage yards, but chances are a used one will probably go bad soon after replacement. Otherwise the best alternative is to just get a new one.
For the SC coupler check ebay or just about any of the 3800 aftermarket suppliers (www.intense-racing.com
) and on ebay do a search with the key words: "Eaton Coupler", "SSEi Coupler", "M62" or "M90 coupler".
Should cost around $70 IIRC for the gaskets, SC oil (use only GM SC oil from your local dealer or sellers on ebay), and a coupler.
Good luck and welcome to the club.