sources

(still being updated)
General parts

Summit Racing - Everything, at a good price.
Pegasus Racing

Kit suppliers
As far as I know, these are all reputable companies. You'll have to make the final determination on that score.

FM Westfield - Miata-based Westfields. A high-end version of my car. I like this place, because I work there.
Factory Five Racing - Cobras
Stalker - V6-powered Sevens
GTS Tuning - UK Locost supplier
Deman Motorsports - Bike- and car-based Sevens and parts. Parts are also sold through Locost North America.
Coveland Motorsports - Miata-based Sevens and parts
Kinetic Vehicles - Locost kits, parts and other Sevens
Canadian Online Locost Distributors - aka COLD, Locost supplier
Rocky Mountain Sports Cars - Caterham Sevens and parts
Champion Motorcar Company (CMC) - out of business.

Electrical bits

Digikey
Del City

Brakes

Wilwood
Tilton