|New Shirt, Old Shirt|
1. Buy used.
Good quality thrift stores almost always have shirts that appear to have been worn rarely for one-quarter to one-tenth of the price of new. Unless you're going to an event for which you would have bought a new shirt, a thrift store shirt will look the same to you, everyone who sees you, and everyone on earth except your banker.
Another reason to buy used in the USA is to help save jobs. Nearly every item of clothes is now made abroad in places where people work for starvation wages, pretty much killing off our domestic garment industry. The many labor and environmental costs of that practice are born by the first buyer, whose money goes abroad to continue the cycle. However, when the item is donated to a local thrift store, the new money you spend on it tends to go to local workers who staff the store. It's not a perfect substitute for a domestic garment industry, but it's something.
2. Buy natural fibers.
Unless you have a special need for polyester or whatever, cotton, silk, bamboo (a very nice fiber!) and so forth will give acceptable results or better in everyday wear. There may be a few situations, such as performance atheletics or safety gear, where artificial fibers have earned an edge, but most of the time you aren't going to be in such a situation. (And of course if you *are* unfortunately in an accident involving fire, the last thing you want touching your skin is something that melts. Natural fabrics will burn before they burn you!)
3. Rag Them.
When your shirt reaches the end of its usefulness as a shirt, you simple convert it into a rag if it's a natural fiber. Cotton rags are amazingly useful for cleaning, and can be used over and over.
4. Compost Them.
When your cotton rage has become too raggy even to use as a rag, you can shred it and compost it as long as it hasn't been contaminated with stuff you don't want in your garden (if you use your rag to clean up machine oil, trash it in good conscience - your health is important!)
I haven't composted a whole lot of rags, but the little bits of rag I added to my compost heap were gone when I turned the heap the next year, evidently processed by helpful worms into beautiful brown soil.
I prefer Hawaiian shirts with coconut shell buttons, mostly for the look, but I'm pretty sure the buttons would compost better than plastic.