There are few 'legal' antibiotic approved for chickens that are used for human egg production in the US... But, IMO don't let that deter you from using 'unapproved' antibiotics just observe a period where you toss all the eggs, generally 2 weeks to 4 weeks after the last treatment...
With that said most farm supply stores will sell Tylan 50 for goats and other animals, in most states you can get animal syringes without a hassle and in many other states you can even buy insulin syringes at the local pharmacy for insulin injections, just asked and the pharmacist and he/she should hook you up with some, don't be shy or nervous to asked, if they give you a funny look or ask if you are diabetic just explain you need them to give your chickens antibiotics... They should only be a few bucks for a bag...
That said giving shots can be intimidating but it's not really that hard, there are several tutorials online that give step by step, probably some videos as well... I inject into the breast at about a 45° angle so the needle is only about 1/4" deep and inject slowly, alternating each side with each injection... Some people prefer to give it orally using a blunt syringe and squirting it down the throat, personally I find giving a shot easier and more precise...
Dosage of Tylan 50 for standard breed chickens is 1/2cc aka 1/2ml orally or injected once a day for 5 days... If you are using a 1ml insulin syringe like those sold for insulin at Walgreens, Walmart and CVS pharmacies it should be marked 1ml on the side and have a 0-100 'units' scale or a 0-40 'units' scale on the side, fill it half way to either the 50 unit or 20 unit mark respectably depending on the syringe... If you are in doubt post a picture and ask as the syringe markings do vary...
Be aware there is also Tylan 200, if you use that the dosage is 1/4 as much or about 0.125cc for standard chickens, since it's much harder to measure in those small amounts most people use Tylan 50 and it's generally easier to find...
There are of course powdered antibiotics you can mix with water, and made for poultry but dosage is to be blunt incredibly hit or miss when mixing with water, if the bird is not eating and drinking normally they likely won't be getting a decent dosage of medication either from the water... This is why I like injecting, it's guaranteed that they get the entire dosage...
Unfortunately all I could find was tylan 200. Since it's such a tiny amt could o add some water and put directly in their mouth?
Yes, you can orally dose if you prefer...
To make measuring easier you can dilute it in a small cup if you want, 1 part Tylan 200 + 3 parts distilled water will get you Tylan 50 dosage strength, I don't know if I would recommend injecting your own dilution as it's not sterile but it will be fine for oral dosing...
You can also just squirt it on some pellets (or a treat) and isolate the bird for a bit an let them eat it...
Most chickens will not refuse a little helping of canned cat or dog food,easy to mix a dose in that, just stir it up and give it to the birds one at a time...