If the hole saw sizes available are not precisely those that fit the fan size, get the approximate diameter that is slighly smaller, and either finish the hole to your satisfaction with a dremel cut-off saw, Radio Shack "nibbler" tool, or a good file.
There are sheet-metal hole-punch tools that you can buy just for the purpose of making such holes, they make the work quick and easy -- a matter of a few minutes, the hole is very clean with no burrs, but the tool can cost at least a few hundred dollars.
Personally, I have been using a Dremel, but it is a lot more trouble in terms of the attention one must pay to the work than a hole saw. If the hole saw fits a standard AC hand-drill, I'd probably do better to put the wear and tear on my Mikita than on my Dremel.