It used to be that this would be one of your most expensive purchases if you're planning on starting your own tree-ring dating laboratory. Not any more! Good microscopes will cost US $400 to $500, which is a great deal considering a few years ago we were paying nearly $2000 for a good microscope. The most common microscope used for tree-ring analysis, whether you're just crossdating or perhaps measuring, is a boom-arm stereozoom microscope. Many companies make these, such as Leica and Fisher Scientific, but today you will find greater variety and higher quality for lower prices.
I'm not kidding when I say look no further. This is now the site where I purchase all my microscopes and I've purchased close to 12 from them in the past five years. Everything you will ever need in a stereo-zoom boom-arm microscope can be found in their Model SM-3B for only US $350. You'll also want to purchase an illuminator, and we use the large-bulb LED model LED-2M-YA. I've also received very courteous service from them when I had minor problems with invoicing. Finally, they ship quickly, so no waiting for months like you used to wait before! One word of caution, however, is that they do not ship internationally, it seems!
If you're in the market for a complete microscopy system (camera, trinocular micfoscope, halogen ring light source, and imaging software), check out their Model SM-4TP-FOR-P for only US $900, but you may wish to consider purchasing their high-end system, the Model SM-4TPZ-FOR-5M, for about $1500. This includes a 5.0 megapixel camera.
The reality is that we no longer have to purchase an expensive microscope from a company with a store front. From this site, you find a variety of stereo-zoom boom-arm microscopes suitable for dendrochronology, with starting prices under $600 (but not as cheap as AmScope above). Their best selling models are the OM2300S-JW11 for US $550 and the OM2300S-V6 for US $660. Their illuminators appear to be somewhat lower in price than AmScope, but I've never tried them out.
Probably the most expensive of the three online companies, here you will find boom-arm stereo-zoom microscopes ranging from about US $1000 for their National Optical 420 model to about $1500 for their model SMZ-168, but these already come with either flourescent light rings or goose neck illuminators included in the price! You can also customize these with high-resolution cameras and they supply all the accessories needed for attaching the camera!