I'd say build one. Some of those grocery store shopping carts are fairly narrow. The ones you purchase, not the carts the stores provide for using to shop their stores. I've seen people get them down the narrow aisles of a crowded bus. Some may be too wide, but there are a bunch of different ones.
Other things is to fit and secure a box of the right width to one of those collapsible carts you use to move luggage. They have a vary narrow base for you to find or make a box with the dimensions you want to securely attach to.
Or make your own platform on wheels and screw a box you made or bought to the platform. Use your imagination for the handles.
