We've tried some more general camps that offer training to everyone, goalies included and had limited success with that. I'd stay away from it.
If you're on Long Island, Dynamic Lacrosse Training runs a really good goalie only clinic. The summer, fall and winter ones are the best because they're smaller in size but those ones book up quickly. The spring one is good also but there are a lot more kids. There are other clinics run by goalies that should be good as well. I want to say D3 or something in the New England area and there is one in PA. Can't remember the names exactly off the top of my head - I could be wrong, we're not traveling to them ourselves ;-)
One on one personal training will get the best results but also cost the most.
In hindsight having kind of tried it all, I would say the best bet is to do a few one one one personal training to learn the basics. Make sure you, the parent are there taking notes. Then you can work with your child on your own time and dime. Get the basics down. At least initially, goal tending is all about repetition and form. If they don't show you - be sure to ask how to do a "ball toss" and do that regularly. It's the equivalent of wall ball for goalies. Very important and can be done anywhere.
Once they've got the basics down they will get a lot more from things like the Dynamic Lacrosse training as well as additional personal training to tune up all the minutia.
If you're on Long Island I can suggest some trainers that have worked out very well for my child. Feel free to send me a private message.
Hope that helps! Good luck!