Where can I buy a clear case for my iPhone 6s?

ebay or similar online stores will still have cases for 6S, or you can use a case for iphone 7 or 8 which will fit except the hole around the camera lens is a little larger. My local discount store has them for £0.99

This one should fit but it is probablymore than the value of the phone

I should also say, cases for 7/8 will not have the 3.5mm earpiece hole

You are welcome. I try to find ones made by Tortoise company on ebay, they do come up occasionally.