A resin model is usually something that a person is producing one by one instead of a huge machine in a factory cranking out a bunch an hour. There is one mold that the person handmixes the resin (a sort of plastic) and pours into. The mold only lasts for maybe 50 models, then a new one needs to be made. You make a new mold using the master model. The master model is usually a model that someone has scratch built themselves or an older model that is no longer available. Since resin casting is expensive, time consuming, and hard work, resin models can be expensive. Another plus is that you can resin cast replacement parts or multiple parts.
Modelhaus.com sells models, they are the best in the business
Other casters are R&R & Freeman's.
Best place to find them are on ebay and model car shows/swaps
check this link out, this guy has a lot, none for sale tho