Get phoning round some local breakers these aren't exactly rare cars so there's got to be one knocking around with an intact rocker cover that means you won't have to mess around.
Edit: Times must have changed, last time I went to a breakers they let you loose and just told you to bring any bits you'd stripped off back and they'd price them up.
that will be my next step but the people i have contacted on ebay who are breakers won't let me buy it seperate