Any of the Mustang places. The Mustang kit is all the same foam and seals needed for a Maverick. Just get the right one for A/C or non A/C. For my 70 and my 72, it was the Mustang 69-70 kit. C9ZZ-18500-B
I used a seal kit for a 1971-73 Mustang A/C box for 90 percent of the seals in my 1970 A/C box. The rest were made up from standard household insulating foam from Lowes.