You want the Polyseal Screw caps found here.
I regularly bottle into 1/2 gallon growlers and use these caps. They are reusable (remember to sanitize). They hold carbonation just fine, and I've noticed better flavor from the 1/2gallon size (ageing in bulk) vs my 12 or 22oz bottles... could be nothing, but it seems better.
I collect up as many growlers as I can, and try to do 2 or 3 for every batch of beer that I make. It makes taking a sample to enjoy with friends much easier... not to mention making bottling much easier as well.