Matthew,
I've been making pens for over 20 years and I've done a little casting getting supplies from either Michael's or Hobby Lobby, not the cheapest but close by. I have also found that theses sources are carrying out dated material in the Castin Craft polyester resin and that makes it a challenge due to delayed curing. We have, maybe had, a large chapter of the IAP that was meeting 4 times a year and several guys in the chapter are into casting in a big way. I can maybe hook you up with a couple of them if you want to communicate your needs/questions. I will PM you with my contact info.
WB