With over 4,000 five-star reviews on Amazon, you can't go wrong.
Promising review: "I bought this in charcoal and lost it on a trip, so I bought the exact color again because I missed it so much. Now I'm back to buy it in more colors. It's thick (because it's a watch cap) but I wear it year round in Dallas or Chicago. It's my favorite hat. The fit is snug to the point that I can sling it back without it falling off and it fits over my afro or straight hair the same and doesn't lose is stretch after washing." —CE.
Get it from Amazon for $9.99 (available in 24 colors).