HAven't had to replace any of them yet so can't help with where to get them. I did remove two from each end so there are two per end left and have had no problem with the bands coming off. Easier for me to get the mics in with only two bands per end as opposed to the four bands per end that the mounts shiip with. Still plenty of support for the mics with only two bands.
If you have a local VW repair shop, see if they have some spare air-cooled type 2 push rod tube seals. They are about the same size as the SSM shock bands and should last longer than the original bands. Another alternative would be to visit your local hardware store/home center and look through their supply of O-rings for a usable substitute/replacement.