Vaginal Ball Adivse Needed

I have several sets for me, but I use them for strengthening because of a prolapsed uterus, so I know what works well for me. But, my mom wants a set for strengthening her bladder. I would recommend to her Ben Wa Balls, but let's be real here, there is little chance she will be able to hold them in over a minute. Does anyone have any suggestions for a first time vaginal ball for her. She is not concerned that they need to be only $10 or whatever, as long as they do what they are supposed to do.