I have been writing grants successfully for my org for the last 10 years. Ours is an intake prevention focus-we fund spay-neuter for low income/underserved pet owners and feral cat caregivers in central AZ. We do not provide direct medical care, so, like you, we work with veterinary services.
I have used Grant Gopher and Instrumental in the past, but found that just doing independent on-line research on what might be out there worked just as well for me (and no fee.) I would also say get involved with as many groups/organizations/forums, etc as you can, as they can be a good source for info as to what might be available.
The grant situation is especially tough now, as the money pool seems to shrink each year as the applicant pool grows; however, diligent research and good writing will still get you grants. I would suggest you focus on the smaller grantors (as opposed to say, Petsmart) as they typically have less demanding data collection/reporting requirements; however, you know your org best, so they may also be options for you.
------------------------------
L.A Nesbitt
President
Pets In Need Action League
Casa Grande, AZ 85130
------------------------------