First things first !
Is your space suitable for a greenhouse? (sunny? level? ). Can you fix it to the ground? If not it might become next door's greenhouse
If yes, yes, yes and yes, then good luck. They do crop up on Freecycle from time to time. Also worth putting a Wanted ad in your local shops, as many people don't bother to advertise if they want to get rid of something. My neighbours got 2 greenhouses and 2 sheds this way.