We have the Pet Gear AT3 All Terrain stroller. It has plenty of space for my bigger girls at 11 and 9 lbs. The front wheel can be locked in place if you decided you wanted to jog for some reason.
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I think this website has some of the best prices (be sure to factor in shipping on other websites where prices may seem lower):
Just Pet Strollers - Pet strollers, dog strollers, and cat strollers.
I've also heard of people finding strollers on ebay that they really like for great prices.
Good luck!