You can use a more readily available (and much cheaper) 2 or 4MHz crystal and a 4013 dual D flip flop set up as a frequency divider. I have personally built a 4MHz version now (using Jean-Yvan's board layout - see below) so I know this works.
What you need to do is stick your 2 or 4MHz crystal in the circuit where the 1MHz one would have gone and take the output of that (marked "A" on the interface board shown on the first page) to the divider circuit which is (click on it for a pict version to print using simpletext)...
Now, you could build this onto the main board as well and I thought you would probably need a bigger one but Jean-Yvan Fradet from Canada found a way to keep it the same size! If you want to see how to do it that way check out Jean-Yvan's modification. The method I devised prior to Jean-Yvan's efforts I have kept below in case you want to do it that way.
There are only two components apart from links, the 4013 and the 0.1uF capacitor. V+ and V- are taken from the main board. The input comes from the Xtal on the main board as well (as mentioned already). If you're clever you'll be able to mount the divider board above the main board using some stiff wire. Then you just need to take the output from this board to the HSKi (Pin 2) input of the mac. You use Output 1 if you have a 2MHz Xtal or Output 2 if it's 4MHz.