22 gauge needles I used to inject in breast muscle seemed not very comfortable for by chickens. I switched to 27 gauge & the chickens hardly minded.
You can get 25 to 30 gauge needles at any regular pharmacy. They sell them for 25 cents or less for insulin shots. You will have to ask a pharmacy worker to hand you the needles from behind the register, but you don't need to have any disease or an authorization from a doctor to get them.
Also the attached syringes are also better than those sold for animals. The plunger moves more smoothly instead of in jumps & spurts. "People syringes" are best for when you are giving medicines orally to your chickens, because you are less likely to accidently squirt too much at a time (which would possibly cause some of the medicine to end up in the bird's lungs instead of correctly swallowed).
I don't recommend giving more than .6 cc Tylan 50 at a time. It makes the bird sore (even if given subcutaneously) and appears to reduce its appetite too much (which isn't good for a sick bird).