One suggestion is to call your county Agency on Aging to see if they know of anyone who has a program to help pay for a walk-in shower. Those showers are expensive if one is taking out a bath tub and replacing it with a shower.
Another suggestion is to have the existing tub have a cut out made so it becomes a walk-in shower.... it is much cheaper doing that.
You may want to check with local contractors about the tub cut option that Pam mentioned above. I had my mother's tub cut down, making it a walk-in tub (without a door, we just blocked the opening with the shower curtain) for $800 total. Far cheaper than the $4,000 walk-in tub and definitely cheaper than the $13,000 quoted to remodel the entire bathroom and make it handicap accessible with a roll-in shower.
My 97-year old mom lives in an independent living facility with a tub with a cut-out. It works just fine. I have a shower bench in there and the only difficult part is stepping in and out. But it's much, much cheaper than a walk-in tub. Just install some good grab bars for her to hold on to when she steps in or out.
Another suggestion is to have the existing tub have a cut out made so it becomes a walk-in shower.... it is much cheaper doing that.