That's purty cool. I think I may steal that quick way to make the top part for the base of my Master Chief Helmet. I'm too cheap and lazy to go grab a used Baseball Helmet.
Bondo is Auto body filler, so look at automotive parts stores. Bondo is a brand name, so depending where you're at it may be called other things... like body filler, or resin filler. What you want is the thick stuff... not liquid resin.