I too have been using Ez-mount foam on all of my unmounted Rubber stamps.
And I have for about 5 years now. I love it!
Of course most of the places I purchase unmounted Rubber stamps that foam comes with the stamps.
There is a place that sells Ez-mount foam for about $2.95 a sheet.
It's called Beeswax. And I know they ship internationally.
Here is the link:
http://www.beeswaxrubberstamps.com/accessories.htm
As far as storage. I still use the CD's to store mine in. and have for along time.
If your Ezmount foam becomes less clingy, Just rinse it with warm water and let air dry.
Over time your stamps will pick up lent, pet hair, dust.