The main thing about Bio-Spira is that it needs to be kept refrigerated before being used. So make sure the store is storing it correctly, and make sure that you don't wait too long before going home...like shopping for another hour, seeing a movie, and having dinner.
Otherwise, bring a cooler and put the packet in there after you buy it.
It's hard to find, as I've heard that Marineland requires stores to buy a proper refrigeration unit from them - since Marineland wouldn't want people buying it warm and it not working, then getting negative reviews of the product.
Once Bio-Spira is put in the tank, then fish can be added and should be added relatively soon to ensure there is an ammonia source to keep those bacteria well-fed. Tanks supposedly cycle within 3-4 days.
If you can't find it anywhere, then here is a place that sells it at a good price - but shipping might be pricy since they'll only ship it fast (but they're in East Tennessee, so it may not be too bad for you):
http://fishstoretn.com/bio_spira.html
Good luck!