from mac stores or mac online, the only places you can buy genuine mac! you can apply for a pro discount which gives you discount dependent on your profession, you can use this in free standing mac stores or order over the phone!
I have a MAC store where I live, so I go there. When I'm in London I go to the MAC Pro store. If you don't have a MAC store near to you (the only place where you can get your discount - counters do not offer the pro discount) you can call MAC Pro on 0870 034 2676 for mail order.