Where is the best place to buy a megasquirt 3 from

why not go for VEMS?

European design, started as a megasquirt redesign, but latest design has been a brand new design ground up, only shares same software with megasquirt now.

http://megasquirtavr.sourceforge.net/
http://www.vems.hu/wiki/index.php?page=MembersPage%2FPhatBob%2FUserGuide
http://www.vems.co.uk/

I'm not at the stage of buying a ECU, but VEMS it will be.

I purchased mine from Phil on ExtraEFI. Good support if needed and mine came with a base map which got the car started straight away.
Beware of those on ebay as there have been some bad experiences. I have seem a poto of some of the solder joints

I got mine from Bill Shurvington. Search these forums and you'll find his email address quite readily.

What's the advantage of VEMS over MegaSquirt ? Never heard of it till now. Seems to be more expensive.