I got one from this seller on ebay
here - It's slightly better than the very expensive lastolite version
Mine was black one side and off white the other - however he is only listing the grey - mine cost £25 as it has a faint stain line on the white side but doesn't show up.
The guy said he was having supply trouble so it may be worth an ebay message to see if he has any of these 'shop soiled' ones left.
I'm sure you aren't against a bit of soiling