You can find human-grade pyrantel pamoate at local drugstores or major retailers such as CVS or Walgreens . It is often sold under brand names like Reese's Pinworm Medicine or as generic formulations.
For animal use (dogs, cats, puppies, kittens), it is available at Chewy and local pet stores.
For analytical or industrial needs, companies like Sigma-Aldrich provide high-purity standards. Common Formulations Flea treatment for 100+ colony cats needed - Facebook