It is obvious that you should give a try to Getpotted. Moreover they opened a new shop at 17 Clifton Rd, London W9 1SY and they have an official website there full with different rectangle planter box for instance. . So, you can check this resource for a wide range of materials, including fiberstone, plastic, glazed clay, or even wood, it's fair to assume that most black square flower pots are sturdy, durable and easy to care. Happy gardening