What can I use to push them back in?

I've always used a big C-Clamp. Like the other person said, often you can just use your fingers (but it's going to take a lot of pressure). You don't need anything special, just a way to give some force to push the pistons back in a little bit.

Also, after you replace brake pads you need to push the pistons back in a bit. The new pads will be thicker than the ones you replace, so you'll need to move the pistons back so you can put the caliper back over the rotor. (I almost always do this by putting a big flathead screwdriver or a prybar between the old pads and using that as a lever to squish the pistons back.)

You definitely don't need a specialized tool for this. If you're going to buy anything, I recommend a c-clamp so you can use it for other things in the shop.