Do you use a vet for other pets?  I'd talk to that vet, explain the situation, and see if they'll write the script.

$165?


Make connections with a large animal vet.  We take all of our animals to a vet who does large and small animals because they are not as "foo foo" and don't charge anywhere near that for a visit, and if you have a relationship with them, they'd sell you something no problem.

They are generally more practical, too.  They do not expect us to clean the teeth of every animal we own once a week.

Our vet is $42 for an office visit.  I never heard of a vet who charged $165 for an office visit, but I'm sure those probably exist.

We trap our ferals and take them in for their shots, get them neutered/spayed, etc.  After that, the vets just give me what I need for the cost of the drug for any I can't easily bring in.  We have one who lives in the house, but no way can we get him in a crate to bring him.   Our vets work out whatever we need for him, to be given at home.

Oh, I have absolutely ZERO problem administering antibiotics.  I know the formula for the ones I keep here (the vet sends home bottles for me, rather than doses just for one cat, cuz we rescue and need it all the time. Keeps him from having to come in on the weekends for emergencies.) BUT...that right there is also the reason for the relationship with the vet.  The vet taught me how to administer.

We not only give antibiotics, we hydrate with Ringer's when they need hydration (think IV fluids, but they are not IV they are SubQ), we give anti-emetic when they are barfing. We give steroids and NSAIDs as needed.  All because we have that kind of relationship with our vet.  And if we have questions about dosing or anything, we just text the vet on call.  

HOWEVER, I think it's well worth the money and time to trap/alter/treat and release the animals at least the first time, for their own good, and so I have that relationship.  

However, if a basic vet visit cost me $165, that would change the equation and how I handle things.  Sometimes I could do that. There have been times I could not.

FYI, it's worth a look at your local farm and feed store, in the cooler, to see if there is ANY antibiotic left there.

Last fall I got a bottle of Oxytetracycline --the last bottle they had--and I still have that.  They were selling out their supply because the law changed and they can't sell it anymore.  You might luck out.  It's worth a stop.  (This is useful only if you can catch the cat and give the cat a shot.)

ETA:  You will find that whatever vet you choose, respects you for getting him altered and cared for that first time, and will be more willing to work with you.  

It's harder when they've been trapped before.   My husband sits out there for hours, earning their trust (only to betray it when we slam that crate door) but not everybody can do that.

@TAG_match

You also need to understand Feline Herpes.

Your cat probably has it.  It's a virus. It manifests (usually) as a chronic "cold and sinus issue"  Like the fever blisters you (maybe) get when you have a cold.  The trouble is, when it flares, it often develops into a secondary bacterial infection.  It really screws with the cat's immune system.

THE ONLY THING YOU CAN TREAT WITH ANTIBIOTIC is the secondary bacterial infection. The antibiotic does NOTHING for a virus.  

HOWEVER:

If you can give a Lysine supplement like these Enisyl it will really help his immune system.  I give it when I see the symptoms flare. Snotty, sneezing, acting like he feels bad.  I give it for like three or four days, and it supports the immune system and helps them fight it.

So he may very well have a sinus infection.   But he may also have feline herpes. (It's everywhere and there is no cure.  All of ours have it.  We brought it into the population in a cat who had ZERO symptoms and had been quarantined for three months.  The vet was convinced she was fine.  She wasn't.  So we live with it.)

Another reason to get him caught and checked if you can, but if you can't, just know there are resources to help him beyond the antibiotics (which you DO need if he has a bacterial infection.)

Okay that's a hell of a ramble.  Did that make any sense?

Doubt they will have at human health food store, but maybe.