When it comes to brands like Gucci and other designer clothes, these are sold under license from the houses themselves or buy distributors. You could apply to become an authorized retailer of their brands. Not following these channels could lead to creating a gray market or black market supply, both of which could be seen as illegal supply of goods. I would be very wary of doing business that does not follow the proper channels.

Each of those items will have different supply sources.

The simplest way is to join the appropriate industry association, subscribe to their magazine and go to their conference/expo. You will meet all the vendors. You can also simply buy their trade directory.