First off, when it comes to bicycle components like Brakes, Shocks, Tires, etc, a Haibike is no different than any other ebike or analog bicycle. For parts like that, I like Universal Cycles as well as Ebay.
For the electric drive system, you have to differentiate manufacturers of the drive system. It would have helped pinpoint better what you are looking for had you provided what H-bike you have or are interested in getting.... And going deeper, say on the Yamaha drive system, you have to know if you are dealing with a PW or a PWX2, PWX3, PW-SE. Alot if not all these parts are available on the Yamaha parts sites. You'd have to google search for what you need. The same can be said if it's a Bosch.
The US distributor for Performance Line Bearings in England services our Bosch or Yamaha motor bearings if needed; along the way, they stock other internal motor parts.
Link:
https://ebikemotorrepair.com/?v=0b3b97fa6688
Ebay, Ridewill in Italy (I've gotten Haibike external battery decals from them with no troubles at all dealing with them) and maybe Ebike24.com stocks H-bike stuff but know that buying from Germany has always been a crapshoot.