Have you tried builders merchants or plumbing DIY outlets? Or does your greenhouse have a manufacturers name on it somewhere? (doubtful I know). Failing all that try visiting a garden centre that sells greenhouses and see if any of them have square downpipes.
Failing all of that you may have to cobble something together like a pop bottle acting like a funnel type jobby at the top of the downpipe so the guttering can trickle water down that way...............
oh dear, if you understand any of that will you please explain it to me. I have a picture in my head of what I mean, but you really wouldn't want to borrow my head ;)