sorry for your loss:( You really did a good job trying to help her. I am sure she was happy for all that time that you had her. We love our bettas but unfortunately they get sick just like any human being, so i guess if we want to have them we need to realize it ,and try to help them the best we can . And not all the time we can save them, but at least they were loved and happy, which is more than they would get in the store.
I am glad you want to rescue another betta.
This is link about disinfection. What she had is contagious and you need to disinfect the tank and everything in the tank. This is the link with tips how to disinfect the tank
And i want to add something to the disinfection instructions. It good idea to let everything sit in the sun for a few days after disinfection to neutralize the bleach. Or just let it completely dry out for a few days.
And also if you don't have the filter in the tank you can just do 100% water changes. Just make sure you acclimate the betta to the new water.