Pygmy Cories in a 5 gallon would be ok.

Any other cory would be pushing it. But, pygmy cories aren't bottom dwellers, they stay more towards the middle and top of the tank, so they might bother the betta.

At the Walmart in Palatine they did have a 10 gallon tank with lighting and filter for $30. Palatine isn't too far from you, Suki Wingy. [sorry, I don't remember your name ]

editted to add:
Don't worry, the bettas colors will brighten up SO much once you get them settled into a nice tank and out of a dirty cup. I've seen the brownest of bettas turn into the most colorfull.