If you prefer buying from dedicated veterinary supply sites, these are reputable sources: KnockOut Flea Spray - Amazon.com

While rarely in physical stores, it is available through their website via third-party marketplace sellers. 🏥 Specialized Pet Pharmacies

Carries multiple listings from different sellers, making it a fast shipping option if you have Prime.

Often provides the best price and features "Autoship" discounts for recurring needs.