I like Stacksandstacks.com -- 15" is tricky, but if you include craft carts or rolling sets of drawers, you're likely to find something eventually. Here are a couple:

http://www.stacksandstacks.com/chrome-3-tier-storage-cart-by-household-essentials#

http://www.stacksandstacks.com/moon-and-stars-see-store-and-take-along-by-guidecraft#

http://www.stacksandstacks.com/supreme-laundry-cart-by-whitmor#

http://www.stacksandstacks.com/3-tier-utility-cart-by-o-r-e-#

http://www.stacksandstacks.com/cucina-culinarte-kitchen-cart#

But I got pages and pages of results from "narrow storage cart," so you can look for more...

check wayfair.com, they have lots of carts and are size-searchable.

have you considered making it a trash drawer if you dont have one already? thats what we did with the old compactor when we moved into this house.

well, if you can locate a cart thats fine. but once you've hauled the trash compactor out consider the hole, check 15" trash drawer inserts at a big box place. my handyman had no problem refitting my hole with a trash drawer, making a matches-well-enough panel for the opening. a 15" hole is a 15" hole, and if you don't have a trash drawer now its your lucky day.