Sierra Madre Collection has reproduction mirrors they say are concours quality.
http://www.sierramadrecollecti...-356B-T5-p18328.html
The same with Stoddards.
http://www.stoddard.com/porsch...dy-trim/mirrors.html
Or NLA.
http://www.nlaparts.com/store/aero-mirror-p-1199.html
These are the places people who frequent the 356 Registry buy.