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I believe that most people build thier own. Check the internet for designs and pictures. Go to web sites that have shoot pics posted, you will see them on there as well.
Costco had a folding cart that looked like a radio flyer, soft sided, folds up to nothing, works great. I use it for general shooting, takes ammo, stapler, targets, gun cases, even has a pair of drink pouches for my water bottles.
If you are looking for a push-type cart similar to the one cmcconnell posted, Do-All makes one and you can get it through Cabela's. If you are looking for a power cart, then I recommend converting a golf cart. Oak Creek Golf and Turf in Calgary does some nice deals on good quality used carts.
I've been searching for one myself. I've only heard of two sources so far. Cabelas for the Do-All and a company Traphappens that shows the rugged gear brand on their website.