Snowlabs; I've done some serious testing of motors lately and have found some rather spicy ones (as this is a new build I am taking on id rather keep the models to myself) - however a fantastic supplier of High power DC motors in the UK (such motors used in cncs and the like) are: http://www.ellis-components.co.uk/

They usually deal with larger orders but if you have exactly in mind the spec you want they are more than helpful. They offer modern equivalents to classically used motors such as the Bosch GPA 750.

Also if you have the cash Parvalux do some spot on motors! I cant say who uses them (sworn to secrecy); but I can say is he is a friend with a world champion title bot and has used them for many years!