You could try your local classifieds ads, people often sell used parrot and bird flight/aviary cages. Just make sure the bar spacing is correct (half inch or less) and that the bars are powder coated or stainless steel. Some older cages have pvc or vinyl coated bars, you would want to avoid those because the gliders may chew the coating off. Also avoid any cage that is made of galvanized steel, as this kind of steel is harmful to gliders. The bars should be sturdy enough to prevent the gliders from bending them apart or breaking them off - test by trying to wiggle them in your fingers BEFORE buying a cage. I have one cheap bird cage whose bars snapped off when I tried to install a water dispenser.
If you can't find a cage with small enough bar spacing, if all else fails you can cover a cage with plastic or vinyl garden fencing/hardware cloth to keep them from escaping. Again, don't use regular steel hardware cloth because it's galvanized. Use zip ties to attach the outer mesh to the cage bars.
Finally there is the option of building a cage. There are instructions on this site for building cages from various materials. The advantage is that you can customize the size and shape of the cage, but it can be a lot of work to put together.