Best Buy still sells new copies of Halo 2 Platinum Hits edition. I just bought a copy last week.
As far as buying a trilogy bundle, no. There really is no need. I imagine eventually one will be made. Unless it had some new content I wouldn’t buy it.
I would if the it came bundled with Halo: Anniversary, Halo 2 Vista, and a complete version of Halo 3 and Halo3: ODST on one disc. I’d be willing to pay $120 for that.