We used to use gatorade sports-style (pull-up, push down) lids. Worked really well, actually... the sports-bottle kind lets you pull up to let the air out, push down and it's sealed again. We put one on each bag.
I went to walmart with a cap and tried my cap on numerous bottles and found that Pine Sol worked best and was the cheapest. The gatorade cap sounds interesting.
Man, just call them up. I did, took 2 minutes and they had the caps to me in a week. I think they charged me Three bucks each for a set of plug/cap combo.
I use drain caps that you can buy from the hardware store and cost <$1. They are more tapered and make a tighter seal than the standard rubber plugs and you don't need a screw on cap for the top. They also have a metal ring you can yank them out with. Also, you can tie string to them to help yank/keep from losing.
Thanks a lot for the ideas. I figured another cap of some sort would fit - just didn't know where to start and figured I wasn't the only one who lost a cap along the way!
Nick, Not sure if I need any of those but thanks anyway...I'll find something around here. Do you still have any pumps for sale? Mine quit last year so I'll have to see if I can get it going again.