www.findaccountingsoftware.com is about as comprehensive as it gets.

You could also look at:

www.capterra.com
http://erp.technologyevaluation.com/

Also knowledgestorm.com, 2020software.com, accounting411.com, software4distributors.com and there are others.

It's very tough to start from a list of 250-1000 vendors. With a few basic requirements like industry, vertical market, company size you can start to winnow your list down to a more manageable number of 25. Many of these sites have long features lists that you can use. Our experience is that these aren't that helpful anymore - to many questions about features that every package has. Focus on your critical processes that differentiate you from your competitors.

The sources provided so far are good. I would also add www.accountinglibrary.com. This site as well as the TEC site let users create a needs definition list that drives their initial screening. Other sites do not. While research is certainly an important step, there are critical steps that must be taken to build a string foundation for project success: management and employee buy-in at all levels, tight project organization and control, participation by all employee levels, and a little bit of sckepticism regarding vendor claims. "Prove it to me" seems to work very well.