www.specialtyresin.com is the one that I use to get my silicone rubber mould making kits, and resin from. This is polyurethane resin, and it's pretty durable, and strong. It paints, sands, and glues easily. Only use resin with resin, and not polystyrene with it. It does create some heat, but not that hot. This won't distort the model, but it can create enough heat that it can make it hot enough that you can't touch it with your hand until it cures completely. It depends on how much resin you're planning to use! Just do it in thin layers, but be careful to measure perfectly so that as you do this you don't get soft spots, or discolouration from having too much hardener in the mix!
~ Cobra Chris