Can you tell me if I should buy the fork seals and brake pads from BMW or are they available aftermarket?
Fork seals ought to be available through a local bearing supply house. Here's the size and, if needed, the BMW part number:
36x46x7.3/8.8 BMW #31 42 1 230 001
Take your removed seal with you to the bearing house. Usually, the seal's dimensions are cast onto the seal's face. Make sure you get the same size.
Unless you're running fork gaiters, I'd suggest ordering new felts, too. These are BMW specific parts. They fit inside the rubber lower slider caps and help keep junk out of the damper assembly... usually.
Monte