CO2 sources

Once again, my yeast driven CO2 generator ran out of steam - and I am 
getting increasingly impatient with mixing the ingredients, sealing and 
re-sealing the pop bottle and having to constantly monitor the thing.

So my question is: is there an equally cheap alternative to yeast for 
producing CO2? [What I really want is a 5lb CO2 bottle for < $ 50.00  :-)]

Let me start with some options I already have considered:
1) don't do it at all (no option for me - I have too much light)
2) natural decomposition (Q: how much organic material would I need to 
achieve a CO2 production similar to a yeast generator? What material is 
best for the job?)
3) titrating vinegar into a saturated backing soda solution (foams too 
much, and I cannot figure out how long it will last, and the same hassle as 
above, now with two bottles)

My apologies if this is in the Krib. I am only on an e-mail account. :-( 
and don't have access to it yet.

in Waterloo where the temp bounces from -20 deg C to +5 within 2 days.