If you are on a budget, your best bet is this adidas Beanie. It's a wonderful product, while not very pricey. Another strong contender for your wardrobe is this adidas Beanie. While the price of this product is not very high, it will definitely attract a lot of attention.
For something in the middle of the price range, consider this adidas Baseball Cap. While the price is very reasonable, this product is an easy wardrobe update.