Get them on e-bay or from a Hummer dealer. Some have take-offs. I got a set of 4 wheels and tires with about 100 miles on them for $500 from Oklahoma City Hummer. I think the price has been driven up by demand since then.
I had mine turned on a lathe to open up the center hold to about 3. 776 or something like that. Hoot posted an awesome set of instructions on dieselplace.com about 9 months or so ago.
The size of wheels -- I'm not sure, I do know that compared to stock, they are the same size (17") and width is the same. The only difference is the H2 wheel has an additional 1/2" of backspacing. It doesn't seem like much, but it makes it a little harder to fit large tires on the truck (37's), and also puts enough arc in the movement of the wheel while steering (when stationary) to make it difficult to steer with 315 tires on it.