Help with my 13 year-old cat!!!?

My 13 yo cat Aruba has lost a lot of weight very quickly in the past year. I would say at most she probably weighs 5 pounds soaking wet. She drinks heavily, urinates a LOT, vomits up a foamy white mucus on a daily basis, coughs (like a smoker's cough), and she loses her balance easily. Here recently, she has begun pooping on the WALL. We got a new litter, and I think she may be protesting the new litter seeings how she's NEVER done this before. Btw, where she poos is no where near the litter box. With all of her problems, I don't know if this is her way of protesting the litter or if this a new health problem. We have no money to take her to the vet, though I have been looking around for very cheap places to take her too so I can see once and for all what is wrong with her. I think I may already know for a fact that she has Diabetes. I would like anyone's input on what to do, what they think is wrong, any organizations that could help, I'm lost!! I don't even know what to do anymore, but I love her so much I don't wanna give up on her. Someone, anyone, please help!!!!