Even if it's possible to buy such an antenna (anybody can build a tower to attach it to, so that wouldn't be the issue, allough you would need to get permission from the authorities) you would have to connect it to your servers that run the necessary software and are linked to Internet and the national phone system infrastructure. In most countries there are only a few companies (like AT&T, Vodafone, Sprint, T-mobile) that have such an infrastructure. So I'd contact them about the options.
Be aware that most 'independent' cell-phone providers use such an existing network; they don't build their own.

Best of luck with your project.

Kees