The Vital Proteins collagen is very rich in hydroxyproline at 2058 mg per serving as well as other amino acids. (primallyinspired.com)
Gelatin is really a rich source of only glycine and proline (along with hydroxyproline, its active form). (pathways4health.org)
Nalawade - Chavan S, Zazula GD, Brock F, Southon J, MacPhee RDE, Drukenmiller PS, Harington CR, Hodgkins G (2014) New single amino acid hydroxyproline radiocarbon dates for two problematic American mastodon fossils from Alaska. (amnh.org)