Hi welcome to Yorkietalk, I've been half-way looking, and only found one link so far. I think several people own one, so maybe they will post. This is the one I found.
Pet Cruiser Bike Basket at Metropawlis Pet Boutique – Unique Products for the Uncommon Pet - Strollers, Carriers, Toys, Collars, Leads, Beds It seemed a little big. I've been meaning to ask this same question.
Have you tried e-bay?