A while back, there was a TT thread that had pics of a repressurization chamber for tennis balls using a corny keg. I wanted to try this, as I have a basket of very decent (but flat) Penn balls that I would like to use for serve practice.
First, I got a $30 used corny keg from craig's list of goodness. It's a Pepsi-style keg, and I got a tap and gas fitting for the keg from my local homebrew supply store ($9 each). The keg holds roughly 60 balls.
Next, I got a length of 1/4" ID plastic tube that's reinforced for pressure ($2) and two hose clamps ($3! for tiny hose clamps?) from my local hardware store.
Last, I got a Schrader valve (bike tube valve, basically) with threads from an auto supply store for $2.
I heated the tubing up on one end with a hair dryer to make it soft, and then inserted the Schrader valve, then clamped.
I pressurized the keg to 30psi. I heard some thumps inside the chamber - either popped balls or collapsed balls. We'll see in a week or so what's up.
One last thing - eye protection!
Pressure vessels + DIY = explosion risk.