That is an excellent idea for a speech!
Your best bet, as some members wrote before, is to stick to true published articles. A strategically placed video or photos will be worth a thousand words, but should not be your only sources, or you will get "dinged" for not using published literature.
So I would go with both. Check out the RedFish article. I would also search online for articles on best conditions to care for bettas. There are plenty. Does not have to mention walmart. Then, once you justify the best conditions for betta care with those articles, you can focus on how walmart DOES NOT meet those conditions.
Agree with above member, definitely leave emotion out of it. And if you want to really wow your crowd, (or professors giving you a grade:)), end with something positive, like how even though walmart should not sell live fish due to their inability to provide an adequate environment, they have a very good selection of fish accessories that can help fish owners (who get their fish elsewhere) take very good care of their bettas bla bla bla:)