To farm Belle Poppy you must go to Port City of Guixu in Wuthering Waves. This Belle Poppy farming guide will show you the farming routes of Belle Poppy and the node locations.

Below are screenshots of some of the Belle Poppy node locations.

Vendor location at Jinzhou Main City

You can buy up to 15 Belle Poppy at this vendor

Credit to WILDMAT7 for latest farming routes.