Without question however and based on my exp with them all of these are easy to contact, affable, forthcoming, and sheer pkleasures to deal with and request info be you a new purchase customer or otherwise.
In no particular order, they are:
BAT
VooDoo Cable
Onkyo
Audio Art
Synergistic Research
Dodd Audio
B&W (US & OFFSHORE)
Art Audio
Phase Technologies
Coincident
Odyssey Audio
Audio Magic
Wilson Audio
Blue Jeans Cables
PS Audio
Running Springs Audio
Sound Anchor
Herbies Audio Labs
Vintage Tubes
Imperical Audio
Benchmark
Apogee
M Audio
Lavry Engineering
Velodyne
Silverline Audio
Pass Labs
McIntosh
Cardass Audio
ACI
Definitive Technology
Im sure there are others which escape me now . As to how long some of these cottage industries at their finest will continue to be prevalent and ongoing concerns is anyones guess. Especially those whose operation is restricted to limited numbers as it pertains to personnel eg., one man efforts so to speak. This latter note however has not been a detraction for me to purchase their products. New or used. Any warranty there goes with the understanding (I assume) that is as good as is the ambition and helth of the designer (s) themself. Naturally the build of many of these items is more than solid and that is a tremendous plus to consider too.