I understand this. I have a contractor with a fair price for installation. But those dealers usually charge much more for the installation.
There is generally a reason for that... just sayin'.
...Even if you have a guy who is a window sub that does great installations on the side(which is never a guarantee) or something along those lines, it is unlikely that he will be able to be found in a few years if you have any problems... I suppose if you know this guy personally, that would be a different story.
The best window manufacturers tend to choose the best window dealers/installation companies to offer their products.