You should be able to find an overgrown area where tall grasses and reeds have grown long in most places, but if you're deadset on buying them online I found this listing on Etsy, but it's very expensive. Most of the online listings are for seeds to grow them yourself, or for pre-made crosses. A cheaper option might be broomcorn, a type of millet which is used to make straw brooms, or maybe miniature bamboo.