If you have a 233 - 333MHz iMac, you need the iPresenter from Power R (www.powerr.com). That will give you a VGA port. The 400MHz plus iMac's have a VGA port already. The 350MHz iMac has no option for VGA.
For the 233 - 333, go for a Focus Enhancement's TView Silver (www.focusinfo.com). For the 400MHz and up, go for the Focus iTView DV. For the 350, there is a USB scan converter called the MyView made by Eskape Labs (www.eskapelabs.com).