Using a stone or baked brick for tayammum

My mother is 70 years old...she is suffering from pollen allergy but the allergy is not restricted to only pollen months (feb-march-april)...The problem is the allergy continues through out the year. She is now even allergic to dust, water etc. The problem increases in winter as soon as she wakes up for fajar and does wudu. She starts coughing sneezing etc. and the same applies for all the prayers. Due to her age, when she coughs, some times urine drops leak out...then she has to go to washroom, do paaki, do wudu again and the problem increases further due to all this.. Again she has to do wudu and coughing etc. Her age does not permit to go through all this process again and again. So someone advised to do tayyamum especially for magrib and isha when she has the peak of allergy. My question is from where to get the paak soil for tayymum? We have a park in front of our home but many animals even pigs roam around there. We can't use that soil and the walls are all painted..no dust on them. What to do?