I know of 2 places in Nashua, NH that sell it. Towers Motor Parts and Pine St. Auto Supply.
Not that far of a drive from Boston. Plus no sales tax...
Towers Motor Parts
305 W. Hollis St.
Nashua, NH
603-598-8899 ( paint dept)
Pine Motor Parts
106 Pine Street
Nashua, NH
603-882-9710 ( paint dept )
I'd call them to make sure they have what you are looking for. Pine has a better selection of the POR products on the shelf.