Many veterinarians, including myself, have applications for an interest-free medical/veterinary credit card called "Care Credit". Those vets that have the applications also accept this form of payment. Additionally, many veterinarians are willing to accept a payment plan. Your dog could have anything from Parvo disease to a foreign body, or more; all of which require tests for proper diagnostics.
Your dog needs to be seen by a vet. You might call your local SPCA or Humane Society for advice on where to find lower-cost medical care for your dog. Owning an animal is like having a child. You have to be prepared to provide more than food, water and love - for a lifetime.