Are there any organizations that help financially with purchasing a wheelchair?

Dodge is right.

If that doesn't work, try going to the organization for the disease or condition that you have that causes you to need the wheelchair.

The first step in the funding process is a statement of need from a doctor or trained therapist. All funding groups base what they do on this paperwork (and it is called different things in different places.)

Usually, the doctor/therapist themselves will put you in touch with the vendor and/or funding agency, but if they do not, the next step is to find a vendor and discuss options with them. They know the local funding options intimately.

If this is not an option, then try the local disability advocacy groups- either for general disability, the aged, or whichever group comes closest to her situation.

If THIS is not an option, hold a fundrasier at a local bar and get a used chair from the local vendor or thrift shop.

The thing is, about 99% of people who medically need chairs can get them covered in the first way mentioned, so there is not a lot of structured assistance for the other 1%.

It's called the federal government. Medicare or Medicaid should cover a significant portion of a wheelchair so long as you buy it from a company that accepts them...Which would be most of them.