Have a look at : https://www.olimex.com
They offer 1 and 2 layer boards (not sure if they can do 4 or 6 layers), and offers to supply very low qantities at a very good price. I have not tried them out yet but I sure will! Have anyone on the board used Olimex PCB's? It seems almost too good to me
I had used it several times. The quality is not outstanding but is quite acceptable. You must be carefull chosing the drill sizes, because if you dont respect what they consider standard they will charge you more.
Be sure to read the tech info on the website.