It's called an air pump.
You can overpay at an aquarium store or nursery.
But, mo' betta go to wally word or search e-bay.

They are about $10.
Don't forget to buy some tubing.
The little airstone is optional.
A rubber band and a weight will work just as well.

Bigger bubbles are noisier, but there are no fish to annoy, and bigger bubbles are mo' betta splashers.
It's the splash of bursting bubbles that does the aerating.

Grow on.

Weeze