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
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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.
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