To be honest you don't necessarily need to use beer gas or nitro. The creamy head is all due to the faucet itself, not the gas it's pushed with. Nitro or beer gas is used due to the need of a higher psi needed to push beer through the faucet. The faucet is 100% responsible for the effect not the gas used.
I used to run on beer gas, but alternately you could run 100% nitro if you carb the beer first then switch to nitro to push beer through your facuet. 100% nitro is easier to find and refill a tank.
Also, you could do like I did in the end. Just run system on CO2. When you want to pour a beer, dial up pressure on your regulator to 25-30 psi, pour beer and then dial the pressure back down to where you normally serve at so beer does not become over carbed. Also be sure to blead off excess pressure on the keg.