I would totally ignore the request and talk about the item instead.
Like this:
"can I have that pink ball?"
"that's my dads therapy ball."
"but could I have it? My son would like it"
"my dad does his therapy with that ball"
"but you have other balls, can't I have it?"
"yes we have others, those are for my kids to play with, but we were lucky to find this one because it's exactly what we need for dads therapy."
...you never even acknowledge that she's asking for it, you just re-state your use of it over and over til she realizes she is being ignored.
I would do that with everything; and NEVER begin the sentence with "sorry, you can't..." because then you would be acknowledging her rude question- so instead, just make a statement that you are using it:
"can I have these frozen fish sticks?" "my family is eating those for dinner tonight."
"can I have this box of macaroni?" "we are eating those with dinner"
"can I have this oatmeal?" "that's my daughters breakfast"
"my son needs a bike helmet can he have this one?" "my child wears that when she rides her bike"
"can I have these potholders?" "I use those when I get food out of the oven"
She will see that you bought these things because you needed to USE them.