Do check the caravan battery carrier fitting before you buy so the 'Leisure' battery has a fastening flange in the right place. Remember you DO NOT use a car battery they are unsuitable.
You could go for a second hand car battery from a breaker, but bear in mind it won't last long. If you are using EHU though, it's not a problem, you are only using the battery as a capacitor to smooth the power from the charger and even a duff battery should be capable of that. Nothing would be damaged by using a car battery they are just not designed for slow discharges and will die much quicker than a leisure battery. If you are on EHU and your charger is keeping it full though, the battery will not be discharged and therefore a car battery would in theory be fine. If you are likely to be using it without EHU then a car battery will probably be dead after the first 3 or 4 uses at best.
When I say die, i don't mean flat, I mean buckled plates and internal damage to the battery BTW.