Cambria Bike has them in 24, but they'll have to order them for you (took about 2 weeks when I did). Since they have to order them, I don't see why they wouldn't be willing to order a 20 hole one as well. Just email them and ask. Very friendly staff there.
I stock the Aerohead 700c in silver, grey and black finish. I keep 28, 32 and 36 hole drillings on hand in all three finishes, and in black I also stock 24 hole. 28 and 24 hole drillings are also available, usually within one week.