Used stuff: Ebay. There are a few shop that sell refurbished or used servers and network devices but it's still pretty expensive and can't compete with ebay prices (+ taxes shipping to EU).
(EU/Germany)
New supermicro (and other server) equipment:
sona.de
They have almost everything that supermicro sells. Ordered there a few times and the items were shipped within 2 weeks.
I liked them really much, but had to cancel my last two orders because they didn't have items they had listed on their site on stock and shipping would take more than 4 weeks.
cls.at
Austrian shop, similar experience like sona.de. They have a lot of stuff listed on their website listed with availability of 3-5 days but after 3 weeks still no shipped items and no communication from them that it might take longer or that there are problems.
Pricewise they are not the best shops, but they have a huge supermicro inventory.