You can get an Ista disposable system at most Petco's for about $50. Replacement CO2 cartridges are $10 each. The system is very cute.
In a 2.5l tank (do you really mean 2.5 liters or gallons?) it would last a while if you can get the bubble rate dialed in slow enough... however with no fauna there's not really any harm in running it higher, other than you'd use more CO2. I have a hard time adjusting the one I have with really low bubble rates.. it's really difficult to get the bubble rate slower than about 1 per second, but it's easier to adjust my Aquatek mini's down pretty low (I have one running just 12 bubbles a minute).