I live in Broward county (near Ft. Lauderdale) so often go to Wes Scott Cycles for parts. Wes usually has the part in stock and he's only 20 minutes away.
When I order stuff, I usually deal with Walridge Motors because Mike stocks parts for my non-Commandos.
For the uniquely Matchless parts I need, the go-to is AMC Classic Spares in Beds, UK.
I've also bought from Old Britts, CNW, Clubman Racing, Norvil, RGM, Andover and ebay
Frankly, if you sell Norton parts, you're dedicated to the marque (and maybe a little crazy), and I've never had bad dealings with any of the vendors.
'73 Norton 850 Commando - 3030xx
'69 Norton Ranger 750 - P11/1289xx
'67 Matchless G15CS - G15CS/1235xx
'61 Matchless G80CS - 61/G80CS/41xx