I went to my local Sainsbury's & picked up some Sodastream 1L bottles (screw fitting) for only 50p each & noticed that there were a whole crate of new gas cylinders with FREE on them. I queried this at the till because I didn't believe it - but it was true - so I took a good proportion of them.
I have no idea if this is a local thing or national but it is worth having a look in your local Sainsbury's to see if they are getting rid of their Sodastream gas cylinders & bottles. Great if you have a Sodastream machine & even if you don't the bottles are great for taking on a Picnic with the kids because they don't leak. The pic I've added gives an idea of the type of gas cylinder that was being given away. (The outer pic was slightly different.)