Product review (3 stars): "3L160" is direct replacement for HAIER WD-0350-21 V-BELT
I was able to find a replacement at my local hardware store (Ace Hardware, though not all ACE stores are the same). Fastenal sells one Manufacturer Part No. 3L160 (Fastenal Part No. (SKU) 0497618).

The part you want is "3L160" 3/8" x 16" V-Belt about 1/4" Thick.

A Google search shows it is used in many garage door openers.

Amazon lists:

"Dayton 3L160 V Belt, 3l, 3/8 X 16 In"

and "General Utility V-Belt (3L160A)" the one I got at Ace Hardware.

and there are other manufacturers.