I would have suggested ACube's website for SAM systems, but I just checked and all motherboards and complete systems appear to be sold out completely! I do hope they get more in soon. Shows that there is still demand around.
Two other systems not mentioned are from Eyetech: AmigaOne XE (full sized MB) and the mini-ITX version: Micro-A1C. I happen to have both and have been happy with them. I would have preferred a G4 CPU, but the 800Mhz G3 on both seem quite sufficient.
Best places to look for any NG machines:
Amibay, Marketplace here and on eab.abime.net. Occasionally, you'll see an advert at amigaworld.net and amigans.net too.
