I just did ten minutes of research for you on Google...let me know if you'll be sending a check or money order.
I went to Xerox's site (figuring that it would be pretty typical for all major copier manufacturers), and sure enough, you won't find any sales data in terms of number of units. That kind of info (marketshare) is competition sensitive, and I'd be stunned if the companies themselves ever divulged it unless they had to. You might go to a generic business site like Fortune or the Wall Street Journal to see if they have some market analysis information about photocopier sales -- although they would be guesses, it would at least be a source that you could point to as backup.
However, if you're looking for an assessment of photocopier sales in terms of DOLLARS, you're in luck. Most public corporations have an "Investor Relations" (or something like that title) link on their website, which contains their annual reports. Xerox's annual report had a breakdown in their total sales with a line item called "equipment," which included photocopiers and things like paper, toner, etc. It's probably the best you can do -- investors aren't interested in specific breakdowns, generally speaking. There may be more information about specific sub-segments in the investor presentations, but I didn't have time to look. So, I think balance sheets are the way to go. (If you've never read a balance sheet before, you can get a quick lesson
here.)
The only other information you'll need to get is to figure out which companies sell copiers, and you can probably figure out the top ten pretty easily with a Google search.
Good luck!