I used to get a lot of my models from modelzone, but they seem to have lost their edge a bit.
Ebay best to get models from now.
Parts, as Ford RS Mad said above Modelmotorvation are top notch for parts, wheels etc.
Remember when they started, just grown more and more as time goes on, good service from them too.
Rarely mod any models now as the metal costs pushed prices high now. 1:18 sunstar escorts used to be £20 - £25, now they at least double that