Here are some links to HO clubs, HO forums and HO suppliers.
HO Clubs
Brighton Underground Racing Network (BURN) - basement HO racing in Brighton
Worthing HO Racing - the premier HO racing club on the south coast
AVHORA - the hosts of the annual Pancake Cup
HO-UK - UK association of HO slot car enthusiasts, including T-jet racers.

Slot car forums
Slot Forum - UK/Euro-centric forum with HO and club sections
Slot Car Illustrated - World-wide forum with HO section and pancake car sub-section
HobbyTalk - US HO forum with loads on pancake cars

HO suppliers
Slots n Stuff - AW and T-Dash chassis, Aurora, AW and Dash parts, plus great service!
BRP BAT-Jet / One Stop Slot Shop (OS3) - T-jet heaven: cars, bodies, spares, hop-up parts and controllers
Lucky Bobs - almost any HO part and car you might want, including AW T-jets and Wizzard T-jet parts
JW's HO Speed Parts - aftermarket T-jet parts, including the F-16 weights for JL cars
Bad L's - good source for AW and JL T-jets
HOSlotCars - another very good source for JL, AW and Dash T-jets
Supertires - the official place for Supertires and Zoomin Pro Series Tuff Ones wheels
Frank the Racer - our favourite Supertires dealer
MaxTrax Scale Racing - the makers of the Rock County Raceway
Trackmate Racing - the very best slot car timing system
Vincent Wheels - Replacement wheels for T-jets straight outta Deutschland