They tend to be pretty flexible about levels of care. However you will be hard pressed to find any vet these days that will allow a surgery without blood work, parasite screening and vaccinations. I think you will have that hurdle to cross no matter where you take your barn cat.
There's a cat vet off of Brookwood and Main. Cornelius Vets are less expensive and have great care. When trying to do on a budget. CAT adoption team I think has a feral spay deal. Also check Oregon Humane Society for their feral spay program.
They have taken care of our pets for almost 30 years. I would recommend them.
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