Reddit reviews AmScope CA-NIK-SLR Nikon SLR / D-SLR Camera Adapter for Microscopes - Microscope Adapter

http://www.amazon.com/gp/product/B005OZ4BME

comes with 3 adapters, dunno what size you need, but it says it fits most

I use a couple of different ones. For my home-built horizontal microscope, I have an adapter that allows me to attach a microscope objective directly onto the Nikon, something like this. I forget where I got mine, maybe Edmund Optical or ThorLabs.

For attaching to regular microscopes, you just need the appropriate adapter, like this. The one you use will depend on what kind of scope you are using, but if you buy a trinocular scope that is meant to have a camera on it, SOMEBODY will obviously have an adapter.

You can even get phone adapters, though the quality will not be as good.