Here is a list of independent Alfa Garages who can fit the Q2 Differential as an upgrade. The Q2 diff is reviewed independently
here.
Jamie Porter at Alfa Workshop has a guide to what cars can be fitted with the Q2 and a brief overview of how it is fitted, this can be viewed
here.
Garages below DO fit the Q2 Differential (in alfabetical order 
)
England
A For Alfa -
A For Alfa - Buckinghamshire
Alfa Aid -
http://www.alfaaid.co.uk/ - Berkshire
Alfacare/AH Motorsports -
Alfacare/AHM - Hampshire
Alfarama -
Alfarama - Middlesex
Alfatecnico -
Alfatecnico - Nottinghamshire
Alfatune -
Alfatune - Merseyside
Alfa Workshop -
The Alfa Workshop - Hertfordshire
Autodelta -
Autodelta - Londonshire
Autolusso -
Auto Lusso - Bedfordshire
N J S Motor Vehicle Services -
N J S - Worcestershire
Peak Alfa -
Peak Alfa - Derbyshire
Perseverance Garage nee C W Lear -
C W Lear - West Yorkshire (fitted mine
)
Scotland
CM Garage -
CM Garage - East Kilbride
McLennans Garage -
McLennans Garage - Edinburgh
Welshland
A!pha Yapps Garage -
***** Yapps Garage - Cardiff
Waterfall Garage - Unit 3m, Newlands Av, Brackla Ind Est, Bridgend, CF31 2DA - 01656 750750 - Bridgend
Ireland
Gerry Campbell - TI Autos - Cowper street, Dublin - 01 8386567 - Dublin
Rest Of The World
Abdulla Ahli Garage in Al Qusais -
Alfa Specialists Dubai - Dubai
The garages below have been contacted and declined to fit or it is unknown whether they will fit at this time.
Alfatech -
Alfatech - West Yorkshire - Declined to fit
I'll update the list as people add other garages known to fit (or not fit) the Q2.
Additionally if you do go ahead get a Q2 fitted please update the poll found
here (Prevelance of Q2 differential).
Sean