Shows which of your connected supplier branches (Zuper vendors) stock each product or variant, for example to choose a branch before ordering. Pass a variant_code to check one variant, or omit it to check all of the product's variants; a branch is listed when it stocks any of them. Availability comes from the per-branch stocking data Zuper syncs from SRS, ABC Supply and QXO, not a live stock count. Unknown or inactive products, and companies with no connected branches, get an empty available_vendors rather than an error. The provider must be connected (403 otherwise). Needs a full-access API key; read-only keys are limited to GET requests.