BringFido.com is a leading resource for travelers with pets
They occasionally get samples and send them to subscribers of their newsletter
Sign up for the newsletter for possible future samples

They are known for doing this but no guarantee on future action

https://www.bringfido.com/promo/newsletter/

From time to time, we receive free samples of pet food, dog treats, and lots of other great stuff from our sponsors. Sign up for our email newsletter now, and we’ll share Fido’s favorites with you too! You’ll also receive exclusive discounts on pet friendly hotels, tips on great places to bring Fido