Jasp, if your current mirror lenses are still in one piece or good enough to trace around as a template, then Halfrauds will cut them to order. They'll take about three weeks to do it though, so no good if you're in a hurry
Halfrauds sell some black unpainted generic ones which are identical to the T8 ones mate - last time I looked in Swindon, they were in the clearance bin for a fiver each !