Affordable or Free Pet Care Near Riverside California?

We have a Boxer and she's about 10 years old. Within the last few months she has grown some type of lump near her rear and it's getting bigger. We've taking her to a vet twice and both times they say keep an eye on it and they could x-ray it and say what it is, but to operate to have whatever removed would be expensive. She doesn't seem to be in any pain as of yet, but here's the problem the operation we could not afford as we live check to check and I very recently was laid off from my job. I was wondering if anyone has ideas as to where we can go near Riverside California. Any advice would be helpful.