Assuming you're in the United States, your best bet would be to contact your mother's caseworker at DSS, Department of Social Service.

Unless it needs to be a special wheel chair, such as a powered one, you may check the local thrift shops, as well. Call around to places. I know it sounds odd, but often they get in medical equipment like wheelchairs and walkers. You may also check medical supply stores and ask if they have any used models that may be cheaper.

Just a thought.

Look in your phone book, usually under Medical Supply, if you live near a medium to large size city, there should be a company that can help you.