Try PumaBuild, they have left and right listed on their website although they are notoriously unreliable at keeping the product range up to date.
Else you'll be looking at talking to members as I doubt anywhere else has a rear arch now.
There's discussions about replica panel production on another forum or as the final last resort a Fibreglass replacement from Pumaspeed although it might be a bit unbalanced with steel on one side etc.
There is a rear arch part in two peices, either full or lower part made by a guy in Wales. Geunine parts were used to make the replacements so the match in theory should be good. Lewis from here made and makes them. https://www.facebook.com/whiteland.rest ... s?fref=ufi These were ebay items you could order I will see if I can dig out some more info Lewis at Whitelands Restoration made/makes them. I have the off side pair sat in my garage waiting to be used. http://www.ebay.co.uk/itm/172503568073
19th September 2008: if you have bought a Janspeed manifold please read here clicky me