i know you wanted live plants but what about fake plants, you can get some silk, sofe plants out there somewhere, they may be a little more money but they will last and you wont have to worry about them, on the other hand,
some good live plantsthat are easy to care for are probally the most famous...java moss.java fern. these just either attatch/float or lie at the bottom.floating plants are useful for bettas too, duckweed like someone said eariler are good but grow fast and soon over run your tank, just cut them back when nessary.what about indian almond leaves, ive heard these are good for bettas, you might need to research these if your intrested as i dont know much about them.i ordered my java moss of the internet and came in such a big bunch i had to devide it between by tanks, it only cost about £1.