The largest US manufacturer of Eurorack Modules is Pittsburgh Modular. They are inexpensive, sound great and are readily available from a lot of retailers. The company also produces a set of pre-loaded systems that make it easy to get started in modular synthesis, and has created an excellent bit of documentation to go with them.
I was hoping to talk to the owner, Richard Nicol, for the podcast; with the help of Jim and Elisabeth at Synthtopia, it came together for this episode. Richard talks about his start as a fledgling builder, his work with other circuit designers, his vision for the product line and his perspective on module creation. It was really intriguing to hear about his ideas, but also reassuring to see a plan laid bare.
An awesome interview, and a great way to understand how a database developer becomes a "modular mogul" (triple-grins to this - wait until you hear about his personal modular system...).
Enjoy!