For small to medium-size systems (the size of most medium/large municipal utilities), I'd recommend either:
Advance Control Systems (ACS) Atlanta or
Open Systems International (OSI) Minneapolis
Both have excellent reputations.
For SCADA consulting to determine requirements and help develop procurement specification, project management, test witnessing, technical consulting, etc., there's no better than KEMA Consulting (Chalfont, PA; (215) 997-4500). Two good consultants there I've used are:
Bob Uluski and John McDonald. John has recently been elected President of the IEEE Power Engineering Society so may not be taking on much new work.