Is it possible to get financial assistance to replace our toilet and bath to safely accommodate our knee replacemet?

As an Occupational Therapist and Home Accessibility Specialist I have done a lot of research on funding sources. Unfortunately there is very little unless you fall in an extremely low income bracket, are a veteran, or have an illness supported by a professional association (e.g. Multiple Sclerosis Association) See the following link for information on funding sources and possible ways to save money: http://homeability.com/the-truth-about-home-modification-funding/

In terms of the walk-in tubs. I am not a big fan. They are not an 'age-proof' solution. My recommendation is don't waste your precious savings on this solution. For more details read: http://homeability.com/a-walk-in-bathtub-your-biggest-mistake/
A curbless shower is the most age-proof solution. For other alternative bathing solutions read: homeability/bathtub-transfer-chairs-lifts-benches/

Get a raised toilet seat that you attach to the toilet. I have one of those for my mom and it's great. She also has hand bars that you attach to the toilet. You can find them by googling "Toilet Safety Rails". All of that is less than $100.

My mom also has a "tub cutout" in her regular tub. She lives in an independent living facility that is about 30+ years old. Rather than redo the bathroom, they just cut an opening about 12" wide out of the side of the tub. It still has a bit of a lip so that the water doesn't come out. We have a bench in there that she can sit on. It works pretty well. Not ideal, but not the thousands of dollars a walk-in tub would cost. You could probably get someone to make the tub changes for $500 or less. Google "tub cutout insert" and you'll see pictures of what I'm talking about. We have a hand-held shower attachment and she's good to go.

Suzt Q is correct depending on the individual a higher toilet may be necessary but it is cheaper to get a raised . taking your walker into the bathroom mayalso help getting up from the toilettoilet seat. You can get one that actually fixes to the toilet so there is no chance of it slipping around

You DO need a higher toilet. My MIL got stuck on her lower one and remained there for 8 hours until someone found her.

PS why do you need a new toilet?. I can see the tub and personally I would swap it for a big shower with a hand held sprayer. I have a hip and knee replacement and can still get into the tub with the assistance of hand rails. I am five years younger than you. As far as the toilet is concerned I know it is hard to get up but could you manage better with a rail on the wall. I found a frame for a couple of bucks at a garage sale. you have to take off the toilet seat and slip the fasteners for the frame under the seat and put the whole thing back together.
You finish with arms on either side of the toilet so it is easier to pull yourself up.
This is a simple task but the toilet bolts can be hard to get off and may need someone younger to get down on the floor which you may not be able to accomplish. I personally was able to install rails and the frame my self but it entailed lying on the floor which was difficult to get up again. I can see many people could not accomplish this but the financial investment was very small and works fine for me. Another thing you can do if you have a commode is take the bucket out and slip the frame over the toilet.

You may be able to deduct the cost on your taxes if you pay any.

Medicare does nothing in the bathroom. Railing are easier enough to get a friend relative install but construction you may try even a highschol prog if the teacher is licensed.

There could be a charitable organization in your area that may be able to help at least with some of the cost. is there a local trade school that may provide the labor if you can buy the materials.

No Medicare and Medicaid will not cover the cost of home improvements. They will only cover the rental of adaptive equipment. Your best bet is to talk to a company like Bath Fitter or Home Depot about a payment plan.