I'll answer this from the perspective of you wanted to begin pumping as fast as possible spending as little as possible.
The most frugal answer, if you are resourceful, is to make one. I even cut the bottom of a 2-litre soda bottle, and put aquarium air hose in the top, sealed around it with polymer clay, and was able to pump my whole package just with my mouth. Didn't cost anything and my balls were almost twice their volume in five minutes since I thought of the idea.
I'd highly recommend making a pump in this manner with whatever you can find right away as it is quite fun to see and feel such immediate results.
Now for a more proper pump.. a private and cheap way of making one, which is more comfortable, will result in the custom shaped pump in my gallery. Get a wax candle larger than your unit, and carve it until it's dimensions are say 1/4" more than your own. more space than you need is better than not enough, which is why my prototype has such a large head area. And so extra water can be filled in there as I warm up in the shower with it.. Next once you have your 'new you' carved out get from the hardware store one of those two-part epoxy plunger things. Actually I bought four or five, all the kind that cure in less than 5 minutes. Use something like a small old paintbrush to spread epoxy on the wax unit. Several layers will be needed because the vacuum will tend to draw the sides inward. Between drying I had a pump made in two days, cost under $20 including the tubing. Once at least two layers are over the whole thing (no gaps) Put it in a pot of nearly boiling water and the wax will melt out. This method allows for a perfect fit, just larger than your base diameter.
I feel kinda silly for writing this up when you don't require something so customizable, but as everyone most likely has epoxy and some tubing at a store nearby this is the fastest and cheapest way I can think of to get started. All the 'real' pumps have breakable moving parts, may not be able to be used in the bath, run the risk of someone else opening the mail and so on.