Oh, man. This is deja vu. I was in your shoes like, 4 months ago, when I got my rats. Fortunately, we caught it soon and we went out and got some tetracycline form a feed and seed store (tractor supply store or whatever you call it). Just ask them if they have tetracycline or the medicine you buy when a chicken has a respiratory infection. It's this yellow powder that you mix in their water.
Most people disagree with putting the medicine in the water 'cause it doesn't give you the rat the right amount, but it worked and my rats got better. Just look up how much to mix into the water (i think it was like one 1 Tbsp per cup--don't trust me on that).
This may not work though. If your rats are really sick, then odds are it might not work. I think the only reason it worked for my rats because we caught it right on. Anyways, if it doesn't work, I'd try to order some amoxicillin online if you're having troubles with money. If that doesn't work, then really try to take them to the vet. Good luck.;)
My past horrible experience with RI.....