We buy the black plastic caps like these:
http://www.homebrewers.com/product/4340/38-mm-Polyseal-Caps.html?meta=GBASE&metacpg=4340&utm_source=gbase&utm_medium=CPC&utm_content=&utm_campaign=4340
They work great for us. The regular caps that came with the growlers are generally pathetic.