There are loads of shops near Watford. Okay, heres the ones I use:
Maidenhead Aquatics - Chipperfield. About 10 minutes drive from Hempstead Road, inside the Wyevale Garden Centre.
Maidenhead Aquatics - St Albans. Just of the A414, on the stretch of dual carriageway just after the roundabout where you get on the M10.
Pets@Home - Hemel. If you come off the A41, down to the traffic lights where the Citroen garage is - turn right there, and its 100 yards along on the right.
Going a bit further afield, the best specimens I've found (and with a pretty good range) are Wildwoods in Crews Hill, Enfield. You need a car to get there and they're pricy but worth it. Depending on traffic, thats about 25 minutes drive from the ring road.
Theres also World of Water at the bottom of the Hempstead Road, but I steer clear of there - whenever I've been in there theres always alot of dead fish in their tanks.