Penny
If you are close to a motor factors or Ironmongers , I’d take one of the good O rings with you and ask them if they have any that size. O rings are nothing special and the O rings stocked by a motor factors should be fine. It will be much cheaper than Fischer. Failing that, have a look online and be careful to get the size right. Places like Halfords, Screwfix, Tool Station and Wickes sell O rings but they are usually sold as an assortment of sizes.
