[Arran]

On the cabinet bottom or the chassis bottom? For the bottom of the cabinet you can find those in most hardware stores or home improvement centers, sometimes even dollar stores. For the corner cushions of Philco chassis Renovated Radios is one outlet, Radio Daze another, since it's a specialty item. If it's the generic large rubber washers there are a number of sources for those, AES has them, as does Radiodaze and Renovated Radios, other source might be McMaster-Carr or fleabay.
Regards
Arran