I called around this morning, trying to find a place that would take payments, to no avail.
We finally ended up taking her to an animal hospital nearby, and applying for credit there. Apparently, despite me being on disability, I have good enough credit to qualify, and so I'll be able to make the payments that way, which is a good thing, because just with the x-rays, before any actual treatment, we're looking at $400.
Hopefully the actual treatment won't involve anything much more than that, and we'll be able to pay it off before the no interest six month period ends. Sigh.