Ronseal.
Does exactly what it says on the tin
http://www.mybeerandwine.ie/replacement-lid-p-341.html
You won't find them cheaper than that unless you buy in the us.
I picked up three earlier in the year for 15 dollars in a sale.
There is a english site too, can't
remember it
http://www.candirect.de/Ersatzteile_1