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I prefer to get my shirts from Goodwill. Their tshirts are $2.09. But they tag everything with a colored tag and every week a different color is half price. So I usually seek out shirts that are the weekly color to score them for $1.05!
So, get your super cheap thrifted shirt. The bigger you buy it, the better. It's hard to find them with stripes, so I'm always drawn to those first. If you really want a pattern, check out the polos. They're more expensive, but if you get them half price, they're still in the $2-$3 range. As with anything you get second hand, check it out to make sure it's not faded, full of holes, or stained. I like to wash mine before I begin sewing.