I have a few favorites, but the best I've seen in a long time is H2O Plants. They do sometimes have stocking issues, but they have fantastic collections of low light/low tech plants that go really well with bettas. If you're feeling adventurous, or want to seriously get into planted tanks, they have that too. They're pretty good at knowing how a plant will do in various conditions. If it started out as emerged, they'll keep it until they see some new growth immersed. My bettas love anubias leaves to lounge on. I've ordered from and visited aquarium coop, and they're awesome. I've ordered from Aquatic Arts with ok results, but they are pricey.
I also have a lot of botanicals that I add for decoration and to add tanins. I love tannin aquatics for that, and they will even curate a betta package for you if you want. I brew two cups of plain red rooiboos tea once a week and add it during water changes. It's really helped to heal up my male betta. I thought he was fin-nipping, and he was a little, but then I found my nitrates waaaay too high. He was the only fish in the tank with issues from it; it's a divided 20 long, but I had shoulder surgery a couple of months ago and wasn't doing water changes frequently enough.
MTS snails were deliberately introduced to my tanks to keep the substrate stirred. Pink Ramshorns were introduced to breed quickly and be fed to pea puffers. My female betta kills these if they wander into her partition in my 20 long.