Try calling a few shelters or the animal warden and see if they can recommend a vet that will treat it pro bono or at a reduced rate.

Even try calling some vets and ask directly. Maybe offer to pay for the medicine and have them not charge for the visit.

If you have to send it to a shelter - find a no kill one.
(but I'm with the camp that says it found you for a reason. If you can keep it, keep it and you'll be well rewarded)