It's not a lot of work to get a Windows/IE virtual environment up and running on a Mac, I use Virtualbox daily in combination with https://github.com/xdissent/ievms This shell script automatically downloads Windows virtual machine images from Microsoft, so that testing can be done in IE6/7/8/9. You don't even need to use separate machines, if you get a Windows 7 virtual machine set up you can use IE Tester to run IE5…
It's actually much more work to test on a Mac when developing on a Windows machine since Apple is so protective of OS X that they don't allow it to run in a vm. Which is sad because I like to have all kinds of users but it's really really difficult to accommodate for all you guys browsing on a mac. Please hit me with hints to make life easier for myself if you have some...
The only thing you'd need a Mac for really is ensuring that your colors/contrast look good, and that fonts are readable (on account of the font hinting).
The other option, of course, is to either buy a Mac, or build a Hackintosh.