Where can I actually purchase this for Vegas or any other NLE?

I don't know of any commercially-marketed version yet. MainConcept makes a DVCPRO25 and DVCPRO50 codec that works within Vegas, so perhaps they will develop one.

The Avid codec can be used within Vegas, but it's a Quicktime codec, and Vegas doesn't interface with Quicktime very well (I can play back multiple DV .AVI streams in realtime on my computer, but I can't even get one Quicktime DV .mov to play back in realtime. Quicktime integration in Vegas is quite slow). So it does work, but it's really slow, and not representative of what Vegas could be doing if it had a native VFW codec.

Vegas doesn't interface with Quicktime very well (I can play back multiple DV .AVI streams in realtime on my computer, but I can't even get one Quicktime DV .mov to play back in realtime. Quicktime integration in Vegas is quite slow).

You must have an older or non-optimized computer, Barry. Quicktime isn't read natively inside of Vegas, it uses a system call to query the Quicktime player for decoding files, but generally, this shouldn't show down a typical QT DV file to the point of dropping frames. DVCProHD will almost surely drop frames, however. It would be great if Vegas could decode internally, but my understanding is that it's a licensing issue.

What if I just wanted to use it as an intermediary codec? I'm going to be editing in DV anyway and just replacing the AVI files with the HD ones when I'm done. Would I be able to use Avid's Quicktime DVCProHD codec then?

I'm sure you could, provided you made sure that each file was frame-accurate identical.

The only other drawback to the Avid codec that I've encountered is that I can't figure out the right sequence of settings to give me proper 720p compression. I've got it working perfectly with 1080i, but for some reason 720p isn't communicating properly.

You can download it and try for yourself; I just don't want people to get the idea that "this is what DVCPRO-HD editing will be like in Vegas", because once a properly-integrated codec is released it should F L Y.