Callon Trading
51-53 Whitechapel, Liverpool, Merseyside L1 6DT
0151 236 8811
Don't know if they're still trading; it's been a while since I paid my first and only visit.
You can Google a couple of others but at least one of them is further afield than the city centre area.
Contact Left is a seriously good outlet but it's a pain to get to:
Contact Left
Altcar Rifle Range in Hightown
Liverpool
0170 487 2258
Again, I've only been there the once a while ago so this is not recent information. Perhaps a call before you go...