I read a post the other day about the arches and someone had the bright idea of getting a pulsar front wing, cutting it off (enough to graft it into your 1/4 panel as you would anyway) the benefits are that you would have the same style lip and the holes for the spats (hopefully these holes will be in the same place!)
So this is a good idea, ive tried everywhere to get replacements aswell and ive yet to find a source, As a panel beater i reckon this is a good bet.
If you want a walk through talk through on how to fit rear arches il note it down for you but its a bit of graft!!