I do love your bags (they are georgous and I love the colors) and you would do best trying to sell them at hospital gift shops and possibly medical supply stores. I like the idea about physical therapy (PT) locations. If you could get an "in" at one of these places and talk to the office staff for ideas or other PT location you might be able to make some money that way.
Different sizes are a good idea too. Have you though about making something a bit stronger than cotton fabric?
I went to this website
http://www.marshalldrygoods.com/60in...oth/cat_3.html and they have 60 inch wide premium broadcloth for $2.29 per yard. Bad side is that they are only in solid colors. They also have some inexpensive quilting fabric as well.
I think you will need to keep your costs down if you want to make a profit at this. Try to do something unique. A lot of people in walkers are usually on a fixed income for one reason or another and will not be able to spend much money.
Have you thought about trying to sell your bags at a craft fair or something along those lines?
I guess it just depends on what you want to do, whether you want to try to make money from this, just want to benefit a small group of people, or whatever?
I don't know if this will help you or not. Good luck!