r/AquaSwap - follow the Title requirements (read their subreddit rules) - please rehome any or all bettas you feel you cannot provide adequate care for. Ask to see photos of the tanks that the prospective adopters will put the bettas (you want to see tanks at least 5 gallons, with live plants or soft silk cloth plants, compatible tropical fish if in larger tanks of 15+ gallons, tanks that look well-kept etc.). If you find people to adopt the bettas, meet them at a safe public location (preferably during the daytime, with plenty of people around as witnesses, and/or security cameras on site) and go with someone you trust if possible or let your friends and family know exactly where & when you are going in case of any safety concerns.

Good on you for caring and understanding that the conditions they have been kept in are wrong and cruel. Change their water daily, at least 50% every day w/ conditioned tap water in the meantime. Feed a small amount of food for each betta daily, remove any uneaten food after 5 minutes (a brand new/unused clean turkey baster will work to spot-clean).

How to care for betta properly: https://www.reddit.com/r/bettafish/comments/3ow6vz/info_betta_care_sheet/

https://www.reddit.com/r/bettafish/wiki/index

Look for at least 3 gallon (absolute bare minimum) tanks on sale at Petsmart/Petco, or if possible, at least 5 gallon tanks (ideal minimum).

Get cheap basic sponge filters on Amazon: https://www.amazon.com/Mironey-Driven-Filter-Sponge-Aquarium/dp/B08F7HF4LY/

https://www.amazon.com/AQUANEAT-Aquarium-Sponge-Filter-Breeding/dp/B09K3N9D9W/

- you'll need an air pump to run the sponge filter - https://www.amazon.com/Pawfly-Aquarium-Aerator-Bracket-Accessories/dp/B0B683DHKT (comes with check-valve for prevent water backflow in power outage) OR - https://www.amazon.com/Aquarium-Portable-Oxygen-Energy-Saving/dp/B08QCMM8DD

manually adjustable heater (for up to 5g): https://www.amazon.com/Aquarium-Submersible-Temperature-Intelligent-Controller/dp/B099N6633R/

r/AquaSwap - may have free/cheap live plants - look for floating plants or easy beginner plants that you could start with. Or Pothos, common vine plants you can stick their roots in the tank water and they will help absorb some excess nutrients from the water.