I made gas tubes using solid plastic tubing (plastruct), available at most hobby shops. I painted them silver and then bend them to fit. You could also probably make one out of aluminum tubing, I haven't tried that.
The Marui M733 and most G&P front kits come with gas tubes, these can be found sold secondhand.
You can also buy gas tubes from makers like King Arms.
Last, but not least, a real steel gas tube is not too expensive either. In any case, you will have to figure out a way to mount it to your gas block and run it up into (or through, if you have a real-steel style) slipring assembly.
It's some work, but one of the things we strive for in Airsoft is realism; I say it's worth it.