Tons of places sell every different size of silicone coupler that you can imagine, just search "silicone coupler" . 4" - 4" is a standard size. 3" - 3.125" is not however. You'll either have to make a 3" - 3" work or an 80mm - 80mm work. Pull the pipes off and measure OD of the pipes carefully, this will be close to the ID of the coupler needed.