from a vet site online- they use a lot of the same meds for humans on pets. ( I've used them before (upon a vet's recommendation) when one of my cats had a gunky eye.)
I'd never recommend using anything on a pet unless it was safe! You can do this- one wraps agitated cat in a blanket and kitty lock them between your legs so it has no where to go.
Putting the drops in one time is not going to do anything. They need to be administered at least 2 x a day for a week to kill off the infection.
Or call your pet store and see if they carry this over the counter antibiotic.
(it works the same- just having ;for pets' on the label ups the cost many times.
If you don't want to try the polysporin. If in doubt a quick call to a new vet (tell them you have a feral cat with an eye infection and if you'd harm the cat by using what you have in your medicine chest.
Your cat isn't feeling well so go ahead and get this started.