For the UK and Europe these are the BIG 4 that I've used.
Subprof in Spain
Bienvenido a SubProf
Apnea in Jersey C.I.
Apnea Online Store
Scubaland in France
SCUBALAND Magasin d'équipement et matériel de plongée sous-marine vente en ligne sécurisée
DB Shop in Spain
Scubastore - Diving Gear and Dive Equipment, Diveinn
Dave
PS As mentioned by Spaniard, Spearo Dave in Southern England is not the biggest or cheapest but has practical in depth knowledge and supplies quality gear.
