Like all Sram HG stuff, rare as hens teeth now that production ended for all their HG's three or so years ago. Ebay maybe the best place to look.
The Swizzbee uses the 3 x 9 Sram HG's so I tend to be gentle with the bike.
For more than 100 years, Sturmey-Archer has been a pioneer in the design of internal gear hubs, hub brakes and dynamo lighting. Sturmey-Archer makes cycling easier and safer.