If these were sold within the UK, then the manufacturer has the Obligation to provide you with spares for upto 5 years after the product has been discontinued or withdrawn.
This is frequently becoming a problem with imported effects which are only sold by one sole outlet, or a handful of distributors. It good be a nice cheap effect, but when something goes wrong, there are no spares, back up or aftersales service available
Damage to mirrors is obviously not covered by warranty, but there really is no reason why the store where you purchased it could not order one in for you - although it CAN take a few weeks or months! especially if they are of Chinese or Far East origin.
The only other way is to look around and see if any other popular scanners which have a similar sized mirror. Then simply buy a replacement(s) from the manufacturer and fit it to your own Rapti