Can't remember off the top of my head what the clamp size is, but I ordered off EBay. Just double check it is full stainless steel version, 316 stainless or referred to as A4, and it's a reputable reseller for Mikalor. The sleeve is usually stainless too, as used on VAG brands including VW, so you should be able to source from main dealer. You don't need exhaust paste. Uses 10mm socket to thicken, but don't overtorque.
WD40 won't cut it. The clamp will be rusted solid. You can pick up Makita or Bosch angle grinder for £50. Handy for sharpening mower blades, and playing hard ball with suspension components that refuse to budge! You could break out the junior hacksaw, don't laugh, think I've got one in my toolkit!
I went along to the easiest VAG dealership to buy one for my wife's VW Polo 1.2TSI 110PS, which was a Skoda dealership, they had none in stock, and on back order at the UK warehouse, this was said to be due to change/revision of that part, so maybe it is finally getting made to survive as long as the exhaust.
I found one of the size I needed on ebay for cheaply, so grabbed that - for future use!
To the OP, if you are just going to buy a new joining section, it does come fully assembled complete with its own clamps, last one I changed, on a late 2009 Ibiza, I just harvested the clamps of an aftermarket one and left the original sleeves on the car - same for my 2000 VW Passat 4Motion many years ago - the easy way out!
Ideally you wait until either one clamp spilts and the other will be almost "gone" so will give way if you abuse it, sounds like a bit of a gamble, the sleeve on my wife's Polo did shock me as it is only 6 years old, I "tapped" it with a hammer and the rust fell off, but it will probably last another 2 winters, my I am now prepared with a new sleeve, though it is stupid just to ignore that and wait for the rattle or increased exhaust noise from a loose joint.