You don't want to use an old plastic milk jug. Go smell one, if you want to find out why.
But, glass jugs are cheap! You can buy Carlo Rossi wine on sale for under $8, and use the wine for spaghetti sauce (the Burgundy isn't bad) and then buy a #6 stopper and airlock from a homebrew store.
I think some apple ciders still come in glass jugs, and do some other juices.
If you're making 1/2 gallon batches, you'll probably want a 1 gallon sized fermenter. A growler size (1/2 gallon) wouldn't be big enough to make the batch and give you any headroom for the krausen.