Mercedes Benz dealers, SLS, mbclassic.de, oldtimer-ersatzteile.de in Europe, Authentic Classics, Bud's Benz in the US.
Reservoir - ebay, this forum, junkyards, but not a lot to seal there, just one gasket (A 000 236 00 80) from Mercedes dealer and other places (just that from Mercedes you do not pay shipping, so price will be similar or it will be less expensive at Mercedes.
Recommend rubber parts (hoses) from Mercedes. Just that - I recently was looking for high pressure hose pump-box and in Mercedes it was NLA, I bought in SLS.
As for the pump - I believe in the US you have a lot of overhaul services (search the forum) if you want to rebuild it. Seal set available at Mercedes (A 000 586 31 46) and other places
More detailed info - if you provide more detailed needs. If you are planning on overall overhaul of the system - I would make a list and start checking the suppliers listed. I am sure there are many others as well.