I don't thinkthat there are any free-ranging herds of buffalo in Texas. So, if you're opposed to high-fence hunts, you'll probably have to look elsewhere.

This place is near Abilene: http://buffalo-mountain-hunts.com/index.php?show=buffalo

They claim to not be "canned hunts", but, that may depend on your personal interpretation. They say its "thousands of acres" so that mitigates against it being called a "pen" or a "canned hunt" in my opinion. But, there are low fences, and I don't think buffalo can jump a low fence, although they can undoubtedly run through one if they want... And, they all but guarantee your success.

Good luck!