Anyone know where I can get this?

[PFG]

I doubt you could get hold of an alpha-build of halo for the mac for love nor money. there is a youtube video of a couple of guys playing it as a TPS, and commentating, but i think they were bungie. Just to confirm, that isnt custom edition, which looks exactly like Halo 1 except different maps and weapons, though you can always tell if the weapons are user-created. sometimes they have Mk VI in some maps, but that is also easily distinguishable from Halo 2, mainly because of the minimal/total lack of bump-mapping.

There are already leaked alpha builds (well, one, atleast). The only problem is that the few people that managed to grab it usually will not even admit to owning it. If you ever did find one that would sell the files (most of these people have files only; it is very rare to find someone with the pressed discs), I would expect it to easily cost several thousand dollars.

Halo 1 debug builds, and Halo 2 Beta builds are easy as hell to obtain, though (and much cheaper).

And you are right, those videos on Youtube are of Bungie employess playing the various builds.