You can source these online at a cabinet supply company or a closet storage supply. Home Depot even has kits for this use.

These baskets are made by a company called Sidelines Inc. out if Az. They only come in widths of 18,24, and 30" ( inside cabinet measurement). If you're slightly smaller you can build out the runner a bit if needed but ideally not more than 1". Another company which has a more retail presence is rev-a-shelf. They'll be the same standard sizes as Sidelines though. Both companies offer rattan, or various metal colours.

Call a local closet company or go to closets.org and fine a reputable one there. They can help you out and point you in the right direction. They come in different depths as well.

Below is an example of the steel ones in a closet.

Good luck!