She also needs them to be higher than the average chair. Lower chairs, or chairs with a lot of soft padding bring her body down so low that she doesn't have enough leg and arm strength to lift herself out.
Just thought I'd add that after BBB's blood clot episode, I went out and got some 4-inch thick firm memory foam and upholstery fabric from the fabric store, encased the foam with the fabric and placed it on the seat of her recliner. This firmed up the seat a bit for her and gave her the additional height she was needing to take the strain off her arms and legs when she tried to stand up. I think it all cost about $40.
Since I know you are also handy with the sewing machine, if you find a suitable chair with a slightly too soft cushion or one a bit too low, you might be able to make it work temporarily for your mom by adding an additional foam pad to it. That should widen up the field of choices for you.