The first step is acquiring a nice small, controllable, portable container of C02. Now if you want a huge tank of C02, you will probably want to go to something like: http://www.airgas.com/ . I wanted something I could toss in a backpack and use in the lab, at home, or in the field.
From the previous instructable https://www.instructables.com/id/Paint-Ants-for-Science/, jgencinc (https://www.instructables.com/member/jgencinc/ ) suggested finding a paintball shop.
In the Atlanta area there is a great one called Classic Paintball
with super friendly people. Simply describe the problem to your paintball people, and if they are good, they will know that they need to remove the normal paintball valve, and install a hand-turnable one. This involves lots of twisting, sealant, and torches. They were nice and did it all for me! You will also need a little hose to attach to the tank. My total was 31 dollars for all of it!
Bring some sample insects to test out, (the paintball people might want to see their new device in action!) and make sure to dunk your new tank underwater to check for leaks.