Just power through it. My artist won't work on anyone who takes pain medication. You could try some numbing cream, I guess, but check with your artist. I just don't think any numbing cream will help with the ribs.
I think most people here are going to say, man up. I’ve never used helpers and have completed most of a body suit, which includes most body parts. Learn how to relax and breathe through the pain. Deep breath in, let out while they are tattooing. Repeat.