I got a pack of 3 fairly long ones from Norfolk (safety) greenhouses, they advertise in most of the magazines, they are plastic and have an aluminium frame. Just had a look, in Grow your own mag...60" long, 13"high 19" wide 2 pack for £26.
If you want me to scan and email the page drop me a PM *edit* - or that would be it in the post below!!
Hope this helps
Matt