Roy and I have been collecting Roger Dean posters for a long time. Sometimes we have duplicates because we've bought upgrades and sometimes because we forgot we already owned some. I just listed some on ebay. They vary from NM to pretty bad (aka much loved). All of these except Demons and Wizards were published by Big O Posters in the 1970s.

Better quality:
Dragon and Tree (cover of Tales from Yesterday tribute CD)
https://www.ebay.com/itm/323498012851

Paladin
https://www.ebay.com/itm/323498034519

Uriah Heep Demons and Wizards reprint
https://www.ebay.com/itm/323498043520

Zcarab
https://www.ebay.com/itm/323498062269

Much loved:
YesSongs Arrival
https://www.ebay.com/itm/323498006541

YesSongs Pathways
https://www.ebay.com/itm/323498017510

YesSongs Awakenings
https://www.ebay.com/itm/323498021414

Thanks for looking!