Should I get an aquaclear 30 or 50 for the 15g?
I plan on sticking an aerator in the filter compartment so I don't kill the fish if I overdose c02.
I wouldn't, you'll simply gas off the co2, it's a fine balance with plants and fish; aerators are useless though unless you bubble in a high rate of bubbles co2, even then that's kind of pointless. Aerator would be good at night when the co2 is off, however, as that's when plants use oxygen and produce co2.
Also a hob isn't recommended for a planted, you want something that isn't going to break the surface of the water to maximise your co2.
Where would I buy something like that? I can't find any sites that sell them.
I don't know about the US, but on eBay there should be similar designs and wattage, either that or go for a beamswork led, they are very cheap and very, powerful