I have Red and a Blue regular Rider Back sheets (not Jumbo index). I picked them up years ago on ebay. This was maybe around 2000-2002, somewhere in that range.
They don't seem to pop up quite so often these days. I don't know how often they get out of the factory.
I don't really have much advice for where to find one, aside from keeping your eye out around any magic sales/auction sites, ebay, etc.