GO to the vet. There are VERY few OTC drugs that are safe for cats. No Tylenol or Aspirin or Benadryl. Rimadyl is not safe for cats.
Call the vets office and see if they will let you pay for the visit in two payments. Can't hurt to ask - all they can say is NO. Call your local shelter - they can probably give you the name of a vet in the area that can accept payments or is at least cheaper.
Cats are EXTREMELY fragile when it comes to medicines. Their bodies are so small that any amount - even a TINY amount - can be fatal.