Many online game server providers will also allow you to choose a game that they don't have on their regular list. So if they only offer the newer games, drop them an E-Mail and ask if they can host Halo as well .
They do??. Not in my experience and I have asked EVERYWHERE
. They all complain that Halo is to CPU intensive or Memory intensive blah blah blah.
I have 2 x 16 slot servers and 2 x 12 slot servers ATM (plus an 80 slot ventrilo server, which currently Mexicans seem to only use lol), however all are inactive because I am currently clanless
. They aren't cheap though. A 16 slot server will cost you around $45-50 per month.
Alternatively you can host your own server from your own interest. First by forwarding the ports on your router for Halo (
http://portforward.com/). Then you can create a dedicated server (you can download it from the Microsoft site). It is a little confusing how to do it at first, but a little online reading to solve all your problem.