In the UK there's
Fusion Hobbies for fast delivery and prices aren't expensive when you add on postage costs for heavier items like the motor.
If buying from abroad orders from Japan and especially the USA will get hit by customs charges, stuff from Hong Kong seems to sail through more often than not. From Hong Kong there's
Stellamodels and
RCMart who usually have huge stocks of parts and fast, cheap delivery. From Japan there's
Banzai Hobbies, who have a wider range of parts from better quality Japanese manufacturers, but their orders quite often get hit with a customs charge so factor that into the cost.