Tylan 50 can be purchased at
Tractor Supply. When injecting, use a 20 or 22 gauge needle because tylan is a little thick. I get my needles from my vet. Yes, inject just under the skin, not into muscle or meat. It is very simple.
If you are going to use this or anything else, you must get the bird off the terramycin first. You dont want to mix meds, that is a recipe for disaster. Terramycin is a broad spectrum antibiotic that can treat secondary infections but will not take care of respiratory infections.
If you are afraid to inject, I would use gallimycin water soluble. If you cannot find that, look for oxytetracycline hydrochloride soluble powder. It may go by other names depending on the manufacturer. I buy agrimycin -343, it is the same thing. These meds are specifically for respiratory problems.
The tylan works in 3 days or less and the water solubles will work in 4-10 days depending on how much water the bird takes in.
If you went to the vet, they would probably give you baytril, this med would be your best bet, but you need a presciption for it.
Best of luck.