I need a rear reflector and also the black trim piece that goes round the wing mirror (not sure if I'll need to buy a whole mirror).

 

Doesn't have to be genuine Skoda parts so is there a good place to buy these online or is Ebay the best bet?

Anyone?

There are stuff on ebay.  However, if it's not original, the rear reflector will probably look different.  It's not that expensive, compared to the genuine, so you could probably replace it as a pair....  What about a breaker's yard?  Some of these places, may well have all that you're looking for.