Where do classic car parts distributors and online stores get their parts from?

I have great overhead and income and have considered starting my own online classic car parts distribution company. I wanted to find out some more information so I can get a better idea as to what it'd be like. There has to be a main manufacturer that most of these current companies use to get parts. I haven't had luck finding it...Any knowledge/ideas?|||They sell used parts they collect from various junk yards or get the parts custom made from small scale manufacturers.