Could try a Sodastream set-up. I'm in the middle of setting one up now. Get an Alco2jet Bottle or the smaller bottle, get the regulator off trademe, needle valve from a hobbyshop, CO2 safe hose, and a DIY bubble counter. After that you'll need a difuser. Depending which bottle you use, it could be cheaper then the approx $70 for each of the yeast orientated Plant Gro systems (considering you said you were looking at getting a couple).
You can start on the smaller bottle, then wait til the larger bottle pops up on trademe, or get one from a Sodastream stockist.