Pills

Antibiotics can be obtained from a vet, your personal supply, a farm supply store (only in some areas), or from online pet stores as "fish antibiotics". Allivet.com is a good store for these fish antibiotics.

Here are some Poultrydvm.com links for common antibiotics you might already have in your medicine cabinet: amoxicillin, penicillin, clindamycin, ciprofloxacin, metronidazole, trimethoprim sulfamethoxazole, doxycycline

Always google the pill's stamped-on number to confirm the name and dosage. Get a pill cutter. Use poultydvm.com for dosage. Search on how to read prescription abbreviations

Prescription abbreviation example: 1 mg/kg PO Q12H
Let's break this down. "1 mg/kg" means 1mg of medicine for every kg of bird. PO means "By mouth". Q12H means "every 12 hours".
You may need to do kilogram to pound conversion if you're using a US bathroom scale to weigh your bird (you can hold your bird and weigh yourselves together, then weigh yourself alone and subtract the weight).
1 kg = 2.20462262185 lb
Note: Rounding to the closest number is fine.
Using the example above, a 4 lb bird will need about 2mg medicine every 12 hours (twice a day).

Note: In my experience, chickens often need to be on an antibiotic for at least 2 weeks before results appear, which DOES go against the recommended period. So if you've given up hope at the 1 week mark, continue administering antibiotics anyway.

A chicken that still has a healthy appetite will glady gobble down a pill, especially if it's stuffed inside a treat like a juicy berry. A chicken that is NOT hungry may need to be wrapped in a towel like a burrito and administered pills by force. Secure the wings and feet the most. Having an extra person around to help hold them is always nice.

If your bird is a real fighter (and there's no one around to help hold them), there's a method you can try that works for even the unruliest chickens.

Sit down on the ground with your chicken (wrapped in a towel), bring your legs up to cradle them, and lean yout non-dominant-hand side against a wall, keeping that elbow high. This gives you stability AND prevents your chicken from backing-up to escape. Open their beak with your non-dominant hand by prying it apart gently at the "hinge" with your thumb and fore finger. With your dominant hand, toss the pill in quickly, or alternatively, guide it in with your finger.

Check the ground and/or towel afterward to ensure the pill didn't fall down instead. Source

Note: Once you earn your chicken's trust a little more, you should be able to pill them easier without relying on a wall to secure them

Other Medicine

If you've no other choice, prescriptions drugs are certainly obtainable from overseas online drugstores. I recommend a certain online drugstore in Vanatu due to its excellent customer service. It's https://www.inhousepharmacy.vu

If this site is ever unavailable or sold out and you need to find a current list of other trustworthy online drugstores, ask an experienced trans person or search a trans forum. They WILL know if the medicine is legit. Sometimes bodybuilder forums also have info on trustworthy online drugstores, as they're regularly buying and reviewing "supplements" from them. You may need to use duckduckgo.com to search for these communities, as they're not the kind of content that google prioritizes. The internet is certainly amazing.

Compounding medicine You may need to compound pills with homemade cornstarch "pudding" to cut down the dosage, which is necessary with large MG sized pills. You'll need 3ml/cc luer slip (oral) syringes for this.

Recipe:
Stir 1/4 tsp corn starch in 1/4 cup water. Cook in a small sauce pan on medium-low heat for about 6 minutes, stirring ocassionally*. The pudding is done when it has a gel-like consistancy (shake the pan; if the mixture sticks to the sides of the pan, it's done.) Cool pudding first. Do not add medicine to hot pudding!

If the pudding is too thick to be drawn by a syringe, mix in a few drops of water until it is. If it's too thin, cook a little longer. *Over-stirring can break down thickness in cornstarch puddings.

The Math:
You'll need to do math to figure out the target dosage, mg per ml. This is mg of medicine per ml of pudding. Keep in mind that a chicken can swallow about up to 1 ml of pudding without a problem.
Example (if treating for ascites): This is for 4mg Spironolactone per 1ml/cc of pudding. Split a 100mg Spironolactone into 4 pieces. Each piece is 25mg. Add one crushed 25mg piece into 6.25CCs of cornstarch pudding (measured with a syringe). Administer 0.50 cc/ml of this mixture by mouth via syringe, once a day.
Note: You can easily crush pills between two spoons

If you explain the situation and ask nicely, the knowledgeable folks on the backyardchickens.com forums might be able to help you calculate this.

Mixture notes:
Only add crushed medicine to fully cooled pudding. Store in fridge for up to a week (I recommend only making a week's supply at a time due to potential spoilage or reduced efficacy). Throw-out any unused leftover pudding. You can try freezing unused pudding for future compounding, but it will be thinner once it defrosts. Do not freeze medicine.