Firstly, you won't get a 6Kg Calorlite anywhere (save old stock - and that's unlikely as they were always in short supply!), they were discontinued in Feb.
If you don't already have a Calor cylinder, or are prepared to pay for the rental charge (or can find a s/h cylinder), Flowgas are an alternative supplier with generally a much better reliability of supply. But check cylinders will fit, they differ slightly from Calor's sizes. https://www.flogas.co.uk/shop/category/all-cylinders
You won't get Calorlite, but a couple of times we've managed to get Calor propane from Lakeland Aggregates, Canal Street, Ulverston (at the top of the cattle market by the car wash).