foam gaskets

I am in the process of changing the heater core in my mustang, 85GT, and I pulled out most of the A/C and heating system to give it a thorough cleaning. I am in need of some of the foam seals for the ducting and foam to cover the recirc doors and inlet doors. Any idea where I can find new material to make my own gaskets? I want to replace the current stuff as it is old and hard to clean. I was hoping for automotive grade foam or something that will last for a long time. Thanks.....Paul