Steam is the only provider I know about, there may be others without my knowledge. Old DVD versions from Amanzon or e-Bay may NOT work because they require a login to a server that doesn't exist anymore, and U-Play, the current existing server may not accept logins from new users that use the login data of the old game owner, so be careful, you may aquire a useless DVD.
I would go with the Steam version, it's compatible with the most important megamod Wolfes of Steel, you would need to update the game to 1.2.0 and apply the mod as for the detailed instructions. There is a fix to apply for the U-Play update, read the TWoS section and ask a question there if necessary. Also on Steam you can buy a bundle including all important SH series for a reduced price I think.

XS