Large cardboard box ?

Try asking at your local saddlery for a box or failing that, anywhere that sells large items like Curry's, Asda or a furniture store.
 
Argos or any supermarket or shop that has delivery of large items like home appliances, garden stuff. Or b&q.
 
Try a removals company- they have loads of boxes in all different shapes and sizes normally.
 
If there is a local storage company near you - SafeStore or Big Yellow Box Company they have perfect boxes for sending saddles. That's where I went after selling my daughter's saddle on ebay - also bought a roll of FRAGILE tape which I wrapped around the box many times - no excuses for damaged goods at the other end :)
 
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