Most LFS wont use media that will "fit" in your filter. They will use professional (VERY LARGE) filters and as such, will be reluctant to sell you media.
If you can find one that has an undergravel filter, you can (usually) buy some of that gravel, which acts as a filtration medium.
The sponges should be out of the water for no time, and out of circulation (i.e. not in an active filter) for as little as possible. Consider 30/60 mins a limit.
Ideally, you will put the filter media (sponges, etc) straight into your filter, but if that doesn't work/fit, squeeze the goo into your sponges, rub them together, mush them together, whatever works to transfer some bacteria from one to the other.
Also, be VERY carefull about chorine. It will instantly kill any and all bacteria, hence make sure all your water is wekk dechlorinated. Secondly, make sure you have a source of ammonia in your tank once you get the media or you will starve the bacteria (usually 12 hrs wihtout ammonia is enough to kill them all).
Hope this helps,
Michele