Stan is correct those are the go-to sources for many parts. For sheet metal/body parts, you probably want to spend some time doing some searches in the upper right hand box, there are threads with detailed recommendations on which available parts from which vendors are best.
You might also search for particular vendors you come across to identify others' experience with them. Some really don't have in stock what they identify or have insane mark-ups.
Unless you can find a part's car or someone has a particular used part for sale, some new body parts will likely have to be sourced overseas. You can always post a WTB inquiry in the Parts classified threads. Many have parts they've forgotten they have or picked up and decided they didn't need them.
While not really close to you, this guy in Quebec makes a few sheet metal parts (not doorskins) and might give you some hints for Canadian sources.
http://www.alfaparts.net/bmwcs.htm
Doorskins are available from W&N in Germany, as you've probably seen.
https://www.wallothnesch.com/en/karosserie-blechteile/bmw-2-5-cs-3-0-csl-e9/katalogbild-41-11.html Shipping is more reasonable than you'd think but you might get hit with duty on bulky orders. The restoration threads are filled with examples of installations of their body parts.
You will see reference to "the Polish" guy or parts, which is here (English toggle on the site):
http://sklep.manufaktura-workshop.pl/index.php/czesci/bmw/
UK folks reference Jaymic quite a bit:
https://www.jaymic.com/
CoupeKing are really busy and probably won't have any to part with, but they've courteously responded to even dumb questions from me.
http://www.coupeking.com/shop/product/sheet-metal/
Good luck!