Heat sealers and soldering iron pencils are a couple of alternatives to using the self adhesive strip on your resealable cello bags and the benefit is the seal will be absolutely air tight - plus you may even be able to get several bags out of each one!
Both methods use a heat source to melt, cut and seal the plastic, all at the same time.
Lay the bag across the clamp and push down. Voila!
To use the soldering pencil, run it down a metal ruler.
There are plenty of different makes and models available for purchase on the internet. Or you may find you already have one in your work shed!