I have found that most foam will loose its integrity when compressed for to long.
The Foam that these sanding blocks are made of work better than anything else I have found. The foam mouse pads and flip flops are made of doesn't work as well, because it doesn't compress as well, and doesn't return to its original shape as well when it is compressed.
A Guitar Center type store wont have the pickup screws, but a mom and pop store that has a repair guy will usually keep them in stock. They are pretty specialized screws and you most likely wont find them at any hardware store.