I google searched "minnow ziplock" and this was a result
http://www.tonydean.com/equip7887.html?sectionid=203

I know what you mean about keeping the water cold. Typically Ill switch out the water in the bucket every couple hours with fresh lake water, and throw a little ice in too.

I still wonder if he wasn't just taking them home to brine them. I haven't ever done it, but I know that if you do it right, the minnows will be very firm, and keep their scales on. And you can freeze them until you're ready to use them.