If you want what real current day cowboys wear.
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Boots (no over the top clasps, spurs, and stuff. Go for something like this: https://www.justinboots.com/)
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Light blue jeans pants
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white t shirt, can substitute a linen or seersucker button down if preferred. Plain or checkered pattern if doing button down.
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western hat
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belt with big buckle (optional)
Source of my authority on this matter: Lived in Texas since I was like 10 or smth