I was able to find GXL wire for a reasonable price on ebay. It depends on how proper you need to be. Finding the exact colors with the stripes won't be easy but you can get close.
There are a couple of places that specialize in British/Euro wiring and you can get close with them but might not be cheap. Sorry, I don't have the links on my phone. Snowbum has links to one or two places, I believe.
I was able to find connectors to match the OEM BMW dash light/accessory switches from Mauser. Some of the other connectors seem to be unique to BMW and not so easy to find.
You can go blind searching for the exact stuff but it's not as hard to just get quality stuff and make it work. I wanted to re-do some things also but opted for just cleaning when it came to the connectors.
Also, the difficult connectors I'm talking about are the plastic housing bits. If you just mean the metal terminals, you can start with Cycle Terminal or Eager Beaver. All kinds of things there to get you going and/or confuse you.