sources

General parts

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

Kit suppliers
This is not a complete list, nor is it necessarily a list of reputable companies. The kit car industry is in constant change and a company that was doing well for a while might suddenly disappear. Do some research and talk to your potential supplier on the phone before placing an order.

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. Theoretically still in business, but with many dissatisfied customers
Kinetic Vehicles - Locost kits, parts and other Sevens. Recommended.
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