A bathroom fitter at three plumbing stores told me you can't get lids on their own the manufacturers don't supply spare lids as it seems you've found out as it's an item that could sit on the retailers shelf for years waiting for someone to break one. So resign yourself to a new cistern.
check your house insurance for accidental damage to bathroom fittings if you are covered have a cistern fitted by a plumber and claim. Of course check your excess that you have to pay.
Bob
April 2010