If you need Internet Explorer, you have a few options.
There's still the old, old one for Mac OS X. But you'd need a PPC based really old Mac. Like Mac OS X 10.3 or even earlier. If you have one of those around, you can install Mac OS X version of IE 5.2.2. Which is ancient and a far cry from how IE is nowadays.
But that version of IE you can probably still download from some archived software site.
Or you can use whatever you use for virtualizing or running your Windows. Boot Camp, Fusion, Parallels, Q, VPC, an old PC. You'll probably want to assume your IE viewers are using something newer than IE 7.
And then don't forget that for testing a site there are a bunch of other browsers and OS out there. Firefox, Camino, Opera, Lynx, iCab, Omniweb, Konqueror... if it's a site well made, it should work no matter what you use.