Eric,
Depends on what is available, but there are four main model makers that I have bought from;
Exoto - as per above - expensive, but good detail (probably only CMC is better - but more expensive again)
Autoart - good level of detail and price
Biante - which is a retailler of Autoart and most of the models branded by Biante are made under the Autoart banner - but these are predominately Australian cars (road cars, Touring Cars and V8 Supercars)
Classiccarlectables - to me better detail and quality than Biante and better boxed - same as for Biante - almost all are Australian cars.
Some of the later Norev models are good, because they are the only ones making them (997 GT3 RS, Turbo, etc).
Craig