Injectors tend to be made by a specialist company (such as Bosch) and are normally standard items which the engine manufacturers buy in. Most diesel service centres should be able to tackle the job; your best bet is to Google your local diesel service centre.
I’ve had good service from a company called ‘diesel Bob’, if I recall, they are in Lancashire.
I remember sending him some injectors from an allis chalmers, he stripped them, then phoned with a breakdown of the parts needed and costs before doing anymore
Any decent fuel injection specialist should be able to overhaul your injector. If you remove it from the engine and put the business end into a jar or bottle you can observe the spray pattern or lack of it and note whether it dribbles.
Be aware that a mishandled injector can force diesel under the skin with serious consequences.
My biggest problem is getting to the local places during working hours as I’m allways stuck at work so I have contacted diesel Bob and he seems very knowledgeable and fairly priced so after spending a couple of hours getting it to part company with the head it is now packaged up. There wasn’t much creak when turned over so I hope this should drastically improve things.