Perhaps only tangentially related, but CBS recently had a story about fashion companies funding business incubators.
When I was in my early 20s, I started a tech company with some friends. We had investors, and those investors introduced us to other business partners. With fashion, the start up costs can be a lot more variable -- and I've seen people start lines through all sorts of avenues. At least in the past, the designers I knew who wanted to create a "brand" needed investors, and started much the same way I did. Wrote a biz plan, got investors, and those investors introduced us to their connections. It's as much about financing as it is about mentorship and networking.
Nowadays, however, you can obviously just do Kickstarter campaigns to raise half a million for a new clothing company. No idea how those people make their connections, but I imagine it's just through traditional networking.