This is likely either a thyroid problem or diabetes. In any event, a vet diagnosis is necessary in order to begin treatment, even if it's something you choose to do on your own.

When it's diabetes, there are remissions in anywhere from 20 to 80% of cats when they are treated appropriately.

While many commercial diets are available and your decision, along with recommendations by your vet, this site is very innovative and informed about diet needs as well. http://www.catinfo.org/#Common_Feline_Health_Problems_and_Their_Ties_to_Diet_

Lisa A Pierson, DVM, has done extensive studies into specifically feline nutritional needs and how commercial diets may influence occurrence of disease and disorders such as diabetes (as well as kidney failure and even cystitis).

If it's thyroid disease (from increased appetite and weight loss it would probably hypERthyroidism - an overactive gland), medication may be necessary, or perhaps surgery to remove a tumor (if that's the cause). You can find out more about this here: http://cats.about.com/cs/healthissues/p/hyperthyroid.htm

Places you can begin looking for financial help in the treatment of your pet

http://www.imom.org/

http://www.help-a-pet.org/

http://www.uan.org/index.cfm?navid=163

http://www.animaladvocacy.net/shelters-rescues.html

http://www.lovethatcat.com/spayneuter.html

http://www.spayusa.org/

http://www.mspca.org/site/pp.asp?c=gtIUK4OSG&b=126801

Don't forget to call your local Humane Society for guidance as well. If you have a Pet Smart in town, they often have the numbers for rescue organizations that may also help, at least with recommendations for where (or who ) to go.