I can't recommend a brand as I'm in the UK but I would advise on you buying the 2-part liquid version and not the aerosol foam.
The liquid foams up really quickly (solid in about 30 minutes or less) and is very consistent in terms of density. On the other hand the areosol takes quite a while to go off and has a tendency to contain some large air bubbles sometimes.
You shouldn't have a problem with heat, the reaction is only 'fairly warm' I would say and it won't damage the plastic but don't get any foam on the outside of the kit, it can stick really well.
Most fibreglass suppliers will stock it; if not try looking for boatbuilding suppliers too, they usually carry foam too as it's used for filling boat sections to aid boyancy.
I have had very good results with Wasco's http://www.taxidermy.com/ WASCO X.R. Urethane Foam
it comes in a two part mix I use it to fill 1/6 scale vinyl figure models. If you use it be careful though it really expands allot. I think it is a great product. Hope this helps.
Urethane casting resin is the non expanding version of expanding foam. They add an agent to stop it foaming, however this can evaporate out or degrade with contact to moisture and that's why old casting resin starts to generate small bubbles when you mix it. Any paint or adhesive you use for cast resin models is fine for the expanding version. One word of caution if filling vacform models with it: Although it expands, it also shrinks slightly again as it sets and cools. This can distort the model if it's "flimsy". Injected kits should be OK.
I wouldn't risk it. I think there's a 5% chance of it working and a 95% chance of it screwing up what you've already got. The density of the foam you mix yourself is not really hard enough to vac form with without it crushing slightly. Even those odds might be over optimistic.