OK, this is NOT a recommendation but.....
You can buy capsules of antibiotics for fish from various vet catalogs. Jeffers Vet is one, Lambriar Vet supply is another. (google for either on the web)
Penicillin comes in 250 mg capsules. It's more than tripled in price in the past 3 years, now around $30 for 100 caps... which is still cheaper than the exact same stuff from the pharmacy.
And if you take 4 capsules a day (Assuming you are NOT allergic to penicillin in any form, which is why this isn't a recommendation!!), you'll knock down the infection from an abscessed tooth in short order.
Unfortunately, most people who do this then decide they've "cured" the problem.. and that's unlikely. (Not impossible, though... I did cure one permanently that flared up under a tooth that had been recently filled. It's been 12 years and it never has bothered since. Saved myself a root canal that time)
As far as OTC pain relief, clove oil is one of the best. Any pharmacy (and probably most grocery stores) will have clove oil, or at least one of the "teething gels" which also numb mouth pain.
In the meantime, taking the Rx strength dose of Motrin (advil, ibuprofen) of 800 mg every 6 hours is probably the most effective thing you can do, short of getting a script for codiene or something similar.
Summerthyme
Oh, and just read what Inky wrote. That can be very true- a couple of times I thought I had a bad infection, and it was just food trapped between the teeth and pushing them apart. A set of dental picks and mirrors is an excellent prep item.