Ah yes.. this is very much from the hack and slash school of luthery.
The filler or 'bog' I'm using is automotive. So you'd need to go to Supercheap or Autobarn. There are a number of different types, but I'm using the lightweight plastic one. It's the most common, but there is also metal and fiberglass ones.. probably overkill for guitars
Its an epoxy, so you need to mix it up in batches as you use it. It takes a bit of practice to work out the ratio's, but generally if you only put a little bit of hardener in, you get longer work times. Also if the weather is hot, it will go off much more quickly. It's about $10 for 450gm of the cheap stuff (what I have) so you get a fair bit of filler for your money.