As DM said you can buy an adaptor from Maplins or somewhere like that but, depending on the rest of your build, you might be able to getaway with just using the 4 pin ATX plug. You will need to make sure that you plug it into the top half the ATX Socket (see Picture).
You won't harm your motherboard by using just the 4 pin plug but you might have problems getting the system to run properly if you are using lots of other components. So it might be worth looking into a newer PSU, that has the correct plugs, if you can afford it.