The person I know who offered readings most consistently at spiritual festivals, where she had a new sitter within moments of the previous one, always had a beautiful, fist-sized, extremely clear quartz point sitting on the table with her. I use different stones for different things, but for reading for different people, I like having a clear quartz with me, too... I also have my black petrified wood to ground me and take me deeper into the reading. The quartz clears the energy between readings.
Jasper is famous for its protective properties... i used to favour dark Jaspers, but recently I found a beautiful orb-shaped, fist-sized ocean orbicular jasper, and it tends to ground, refresh, clear and calm energy... I love it.
Shungite is highly regarded as an energetically-protective and cleansing stone... Maybe it will suit you
I've heard of fake ones being offered, though, so make sure you buy from someone reputable...
It really is a personal choice, though. If a particular stone doesn't offer you the courage, grounding or etc that books suggest it will, don't use it... find something more personally relevant.
White Sage smudge... I use it often... it both clears energy and grounds me, and leads me into 'that' space....
Mainly, though, do a check-in with your auric strength... You are opening yourself to other people's energies to best 'read' them, so you want your own energetic or auric boundary to be vibrant and intact... Visualise your aura as an egg-shaped bubble of light, radiating out in every direction a few to several inches from your physical body. If you can 'see' it, check for any open or patchy areas, and energise those to strengthen them... See your auric 'egg' as whole, vibrant, and glowing with light, bright energy. By doing that, you'll radiate your own inner light, attracting lovely sitters, and also reduce the likelihood of outside interference.