You'd need to be DBS checked as you'd work with some officially vulnerable people, and concerned relatives would want to know you weren't a con artist or thief.
Lots of elderly people have medical issues affecting foot/lower leg health (peripheral vascular disease, heart failure, loss of muscle tone and poor skin integrity) which make them inappropriate candidates for massage.
I would get in touch with care homes, nursing homes, assisted living for the elderly in your area, go in person and ask if they are interested in your services, you will need to get a CBR check and do a course (these can normally be done in a day) in infection control, general foot and hand health knowledge of the elderly. The staff will be able to give you back ground information on their residents which will be a great help to you.
I work in a home and a lady turned up and did what I have stated above, we agreed to give it ago 7 months later she still comes about every 5-6 weeks, she now has several other homes she visits.
She does hand massage and paints their nails and the same with their feet, male and female 1-2 do not like to have it done but the other 20 do, she also brings a foot spa which some love to have.