check
REI.com: Outdoor Gear & Clothing for Spring Fitness, Cycling, Camping, Hiking & More.
they had a number of packs and if there is one in your area you can actually bring your dog in (until April 5) and have em try it on. If one is not in your area, just order it on line. Make sure you do the measurements of your dog cause each pack is a little different. We tried 4 or 5 until we got the correct fit.