small and cheap is taking the hose from your co2
source and feeding it into the impeller of a filter.
alternately, you can use a piece of large clear tubing (like that found on a gravel vac) and a small pump like a microJet.
setup the large tube so only the bottom is open, and feed the co2
into an airstone located near the top, then mount your small pump to the side of the tubing so it blows on the airstone