generally 3/8" gas line is used the same stuff used by many as beerline. any brewshop online or ebay will sell it.
if your gas connector has a John Guest push fit fitting, its 99% that its 3/8" line your after.
if your regulator is like mine its got a hosetail to slip the gas line onto, if so use a jubilee clip/hose clip to secure the gas line in place.