AFAIK, the aluminum LIM gaskets did not even come out until 2005. I know our 2004 LeSabre had plastic (I replaced them)--although that also had the plastic UIM.
The aluminum LIM gaskets can be had at the dealer, or perhaps more easily at zzperformance.com or intense-racing.com
Everything else should be obtainable at local auto parts store or at rockauto.com