This is one place. I've bought heat packs here. www.superiorenterprise.com My son works at McDonald's and collects them from me. He would never remember to get them(hes 16) but he loves when they ask what they're for. If you have any restaurant connection where they use eggs by the case just threaten to let a few roaches loose if they don't help you out.
Generally the cork boards used are 12 inch by 12 inch squares. They are much easier to use and clean than the egg crates. Oriented vertically they allow for a thermal gradient. Also easier to adjust the spaces between levels than egg crate. Easy to make and economical in the long run compared to continually replacing the egg crate and the roaches seem to prefer it given the choice.