"Eckcells" offers all sizes and makes of mainsprings for American Pocket watches.
They are located in Stoneham, MA and also operate a storefront business called; "The Watchmaker" in Stomeham Square.
The business is run by Jack Kurdzionak, past Pres. AWI.
Thay can be reached at;
www.eckcells.com
or by phone at 781 438 6977

I just bought a 30 mainspring assortment of sz 6 - 18 for 5 different brands from them at a cost of about $6 ea after shipping.

It is nice to know that new springs are being made and are available. I hope that replacement staffs and stems and crystals remain a profitable biz too, at least until after I retire.
-Cort