Nope, they aren't specially made for macs. They are known as Torx screw drivers and you can find one at the Home Depot or Lowes or any hardware store. I bought a small torx driver from home depot for about 6 dollars and it has every size bit I needed to open a powerbook and an iBook. Just ask for torx...They come in a number system. The lower the number, the smaller the bit size. You probably could find the sizes you'd need to open your iMac up some where on the internet in a take apart guide or something.
Good luck