In my opinion, the Mel Dutton antelope decoy was the best ever made. It's too bad he quit making them. You can sometimes find one on ebay or random classifieds on the internet. If you find one, snatch it up quick, because I'd bet it will be a while before you find another one for sale.
Guys:
I had the same problem about two years ago when I started archery antelope hunting. I looked all over for a Mel Dutton, but like Adam said, you can't find them anymore. I know a guy that knew Mel and he also believed they were the best made. At first, I tried the Montana silhouette decoy, but didn't like the way it moved in the wind (I hunt in SD...there is ALWAYS wind...ha).
About that time, I found that Flambeau came out with their version of the Mel Dutton called the "CommAndelope™". I have one of these and we have called in several bucks in the past 2 years (tagged one and missed one). I would strongly recommend it. I looks very realistic from mid-to-far ranges. Its not even bad up close, really. It can be setup or taken down very quickly (maybe 10 secs). It is light and probably could fit in a larger size pack (or strapped to the outside of), but its carry handle makes it really easy to "carry like a briefcase". I can't remember what I paid for it or where I bought it, but it was reasonably priced I felt. You can see what it looks like on Flambeau's website (scroll to bottom):
http://www.decoys.com/big_game/dcoy_big_game.asp#antelope
Also, I recently bought a couple Carry-Lite antelope decoys. I don't really consider these "mobile", per se, but they could be mobile enough to take them down and carry them on your back and go a mile or two to a blind setup. I bought them with the intent of using them around a ground blind setup when I am not being aggressive and going to the antelope with the CommAndelope (depends on time of year and maybe even time of day). Also, it didn't hurt that I got each of them new from Cabela's for $60 as they were discontinued for the new model (old model was $160). Let me know if you have any questions and good luck!