GM / General Motors: Injectors cleaner

I have a 1992 Transport with 3.8L engine. I have not clean the injectors
since new. The vehicle has 150,000 km
What would be the best way to clean injectors? I don't care much about
injector cleaner solution (add solution with a full tank of gas) such as STP
from Canadian Tire or Walmart. I was told that I can not buy the same
injector cleaning solution from garage so I must bring the vehicle in for
service, is that true?

Quang

Repair shops can inject straight cleaner through fuel injectors. The fuel
system is bypassed. The pour in types are good maintainence, but the hook
up direct
products produce the best results.