If you are a DIY type an excellent source for parts is http://www.hobbycity.com
They cater to RC hobbyists so don't expect much advice or support for ebike applications. But as RC has matured they now carry lots of products of suitable capacity and durability for ebike use. I haven't yet tried their motors, controllers or batteries. A few ebikers have with varying degrees of success, but high rpm RC motors are not easy to adapt to a bike. Their LIPO batteries are starting to look increasingly attractive as the larger format 5AH cells are getting better and cheaper.
I regularly buy from them connectors, wire, shrink wrap, wire mesh guard, etc. And I really like some of their watt meters/power analyzers, battery monitors, battery chargers, power supplies, etc. Especially love the iCharger208B and the Junsi celllog-8. They are in Hong Kong and I'm in the US, but I've found them about as easy to order from as local suppliers. If the products you are ordering are fairly light the shipping charges are reasonable.
As for connectors, I used to be a big fan of Andersen powerpole connectors and bought the 45 amp version in large quantity. I still like them but don't use them nearly as much as I used to. I never bought the high end crimping tool for them and perhaps if I did I might like them a little better although I'm not certain. With the cheaper crimper I have it is a little bit tricky to make a secure crimp without deforming the connector. I only got really consistent results with 12 gauge wire. With heavier or much lighter wire I had more issues. Sometimes after crimping I have to tweak the angle of the blade connector a little to get a snug fit in the housing and make good contact. I eventually settled on a process that involved lightly tinning the wires, lightly crimping and then soldering which gave me good results. But even so I find that over time the quality of the connectors degrade and the contacts loosen and the resistance increases. In my use I find them typically good for about 1,000 connect/disconnects before needing replacement.
I mostly switched to using the RC connectors, the polymax 5.5 mm aused with 10 gauge silicone wire are now my favorite for power connections between the motor controller and the batteries and motor. They are not quite as convenient to connect/disconnect and you need a method to prevent accidental shorts, but they provide a better quality connection. They are solder only, no crimping. I find them much easier to install/service/replace. Btw, I protect my connectors from shorting with little boots over the male connectors and shrink wrap just slightly extending over the tip of female connectors. I make the boots from short lengths of narrow bicycle inner tube cable tied on behind the connector such that the boot extends past the connector tip by ~1/2" of an inch.
My second favorite supplier is Digikey. They are pricey, unlike hobbycity. They have a great selection of electronics parts and ship small orders fast and cheap.
My favorite supplier for motors was Kenny of Crystalyte, but I haven't ordered from him recently and don't know if he still deals directly for orders of a couple dozen motors at a time or not. He was never great to deal with as orders were often slow to be processed and shipped and were rarely filled 100% correctly. But the prices were good.