PLEASE read the cycling thread BEFORE making a trip to the fish shop again.
Mature media from the fish shop would be great, but only if you already have the media you want in your filter, and have a supply of ammonia ready, (which it will be HIGHLY unlikely the fish shop will stock, they'll more than likely try to sell you a couple of 'hardy' fish and some miracle-in-a-bottle-potion).
Correct Steps:
1) See whats in your filter, if your not sure about the media describe it here, (pictures even better!), and people will try to recommend hat might be best for you.
2) Get the media if needed.
3) Get the bottled ammonia, as said by waterdrop this is more likely going to come from a hardware store rather than the LFS. Not sure if you are in the US or UK, but if you are in the UK I've recently seen the ammonia sold just as 'Household Ammonia' in the 'cleaning' sections of both a LARGE local Boots store, and also the local Homebase store.
4) Start fishless cycling, and do not give up, it really is worth it and it will give you a lot less stress and work than doing it fish-in. Also although it takes time it's time that can be well spent choosing what plants / fish you will eventually want, tweaking your layouts, routing electrics etc.
If nothing else it will serve as a reminder to you that your test kit DOES work when in the following months they always seem to report their zero readings for ammonia and nitrIte levels
Good luck!
