The knobs are in the Nuts and Bolts isle of Home Depot in the specialty area where all the Specialty cap screws and such are in the pull out box drawers. These knobs are above the drawers in little plastic drop down pull boxes.
There are several shapes and colors. They cost around .85 cents each.
The package calls them KNOB_TEE 5/16"...UPC # 0 3069978178 2.
There are also round knobs in various colors.

SKU # 562-514