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).
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.