I've been having the same difficulty finding a stand-alone copy of MI2 SE.
Initially, Direct2Drive had a bundle available of 1 & 2 SE, but after they were acquired by Gamefly I don't think they're available.
If they are available, then the games would require Gamefly's game client. If you have to pick a game client (Steam, Impulse, Gamefly, Origin, et cetera...) then you may as well buy the Steam version as they have the biggest library and are the most well known. At the very least, you can play it.
You just want to purchase the game you'd like to play without being online at all times or signing up for a service and agreeing to a EULA? Preposterous!