Links and information companies for the following.
Easy to work with. 4mil pads, 3mil trace/space, enig: https://www.protoexpress.com/
Haven't tried, but advertise advanced specs: http://www.streamlinecircuits.com
Note that advanced circuits has a list of PCB Assemblers:
http://www.4pcb.com/index.php?load=content&page=index&page_id=137
The only pool service (low cost) is from Eurocircuits / PCB-Pool:
http://www.pcb-pool.com (select "Chemical NiAu")
http://www.eurocircuits.com/index.php/pcb-prototype-and-small-batch-services
Sunstone has moderate costs for low-tech ENIG boards:
For high tech (small trace/space), thin/flex, the premier board houses in the states will do ENIG, including:
Sierra Proto Express and others.