Just get some tubing that fits over the faucet spout. Then get some small bore tubing that fits inside that. That's what I use for growler filling. Total cost: $0.00 as I made it up from scraps of tubing I had lying around.
Or you can use a beer gun if you have one of those for bottles. The advantage there is that you can put on a long 'choker' and control foaming better.