There's a company in Bargdner, Montana called Elk,Inc that has excellent products. I use several of their calls and have been very pleased with the videos I have purchased. Check out http://www.elkinc.com/VIDEOS_vta.aspx

Check gun regs in CO and make sure to have proper caliber. Here in Wyoming you have to use ".23 cal. or bigger" for instance. Hunting lopes is very different from what you've been used to.
First, you are out in the prarie and can literally see for miles. So can your prey! Lopes have 8 power eyesight and are very skiddish. L-o-n-g shots are the norm. I'm talking 200 yards+ are not at all uncommon. As a former Antelope guide, the one piece of advice I could give to a "back east" hunter is to practice long shots and get good at it before coming out.
Also, regardless of how you get here, always re-check your gun when you get there. Put a couple in the paper just to make sure beforew launching lead at the game. There seems to be a potential difference in the shot when you change altitudes, humidity, and air density.
Which caliber is for another post. Just don't bring a 12 GA with deer slugs or a 30-30 (lol).