my son in law was lent one by the health people I don't know the name but it screwed into each step ,ok if you were using it but it only left half a step for people walking up and down .
Depending on the weight / Height of the person that needs it Kazal, this company can advise you as they deal with mobility, Millercare, Accrington Lancashire, Google it.
You can purchased used Stair Lift Electric chairs Kazal, there are sites that sell but be careful of 2nd hand, if that's the route you want to take and see one, ask many Questions regards the Movement UP/ DOWN, safety ON / OFF, recommended weight, where it was made, are parts available, EG/ New hand sets for usage, does the seat twist /turn & does this movement work.