I need interior parts for 91 grand marquis
 

Any leads besides junk yards? Like front passenger seats. Complete reat interior door panels, stuff like that.

Junk yards are great -- for junk.
For quality used parts, there are Auto Recyclers all over the country.
The smart guy shops at Recyclers in Arizona and New Mexico -- very dry there, and the parts stay in good condition for a longer time.
Interior parts, like door panels, will be in MUCH better condition if you get them from the desert areas.
Good luck !!

Try eBay I've seen whole door panels and other various interior part there but if you have the seats I would just get them reupholstered