
Would you like an inexpensive, easily converted, cotton/stretch, machine-washable, short-sleeve shirt? That looks like a normal shirt when you take the collar off, no less?
Like, say, this?
Ok, it does have poofiness at the shoulders. Get over it. This is a great investment!
Here's how to do it.
Go get this shirt at Target: The Mossimo Pintuck Henley. It was $12.99 last week, but this week I can't find it on the website, so you better hustle. Buy several. It's got a nice band/mandarin collar style. Hurry.
Bring it home. Sew a button on the outside back of the neck, just where there's a buttonhole on your 'official' clergy shirts. See this photo below. This is a good way to use up small buttons. No one's going to notice it, so use colors or whatever you've got on hand.

Got that? Good. Now sew a button on the INSIDE of the FRONT of the collar, as high up as you can get it, just above where the outside-facing top button is (see photo below). I'd recommend using very small BLACK buttons. That way, when you wear the shirt as a normal shirt (sans white plastic collar), the button is less noticeable.

In this shirt, a size SMALL takes a 14.5" collar.
Those are the best instructions I can give about converting shirts in general. You can use these same instructions to convert a basic button-up shirt, as long as you rip out the fold-down part of the collar and sew the seam back up.
The real trick is to put that front button as high up on the neck as you can get it; normal (non-clergy) shirts just aren't built to ride that high around one's neck, and for it to look presentable (and to avoid making the plastic collar into weird shapes) it's best to accommodate it with higher button placement.
Very important technical details.