In my personal and professional experience, the commercial packaged "potting soil" has all manner of nasty (expletive deleted) in it that you
don't want your body exposed to.
If by "fake" bog you mean something indoors in a bathtub or water trough, then clay or peat would be a good choice (remember not to flush the clay and peat down the bathtub drain and clog the entire neighborhood's sewers, or ruin your septic system).
If by "fake" bog you mean something outdoors, a cheap method is to use what's already there. Get a few 2x4s and a roll of hardware cloth and make yourself a grizzly:
http://www.homeimprovementsdepot.com/ho ... th-wheels/Dig your pit, screen all the rocks and debris out of the soil, fill pit 1/2 with water and put the screened soil back in. Unless you have unusual soil that is pure sand, pure gravel, or mostly rock, this should work well. You can always add a bale or two of peat to the mix to improve your bog.
Oh, and remember- CALL BEFORE YOU DIG! The neighbors tend to get upset when you dig into the gas main (BOOM!) or cable TV. Plus, if they mark out the underground utilities for you and you hit them somewhere else, you're off the hook.