Hi.
Not sure what size of cylinder you want, or what brand, but I picked up 2 x Repsol and 1 x Cepsa 15kg types recently from a scrap metal place in Corbera. Price was just 6 euros each, which was great when compared to the normal price of between 15 and 25 euros.
If there is a scrappies or metal yard near you then go check that out first. Good luck.
PS. People often sell them at car boot sales too. ;)