It's a little expensive, but extremely heavy duty and very well insulated. You have 360 degree protection, about 3/4 of inch thick. Fits the standard 64 oz growlers perfectly and can actually fit 750 growlers like the ones from HF or Treehouse. It's a little snug up top, but does fit. It also fits the 32 oz growlers from Tired Hands.
If you're looking for one to fit two then the Firestone Walker bag would probably be a great option. I have it, but only use it for carrying bombers and such to tastings. It can fit two 64 oz growlers side by side.
i have one similar to the kegworks one from local brewery Greenport Harbor and i love it. it is tall and can hold a 4 bottle bomber in there easily.
like this:
I really like this bag because it can hold 2 growlers and is a bit taller than the FW bag and holds bombers reasonably well. i can keep a growler and a 4 bottle tote in there: