Each spider has a spider tattoo with their member number tattooed somewhere on their body. Shizukus is on her hip, for example.
As they are thiefs, anonymity is a valued good, so I can't imagine them going to a tattoo parlor. At least not without disposing of the entire staff afterwards.
Does one of the members take to tattooing? But how does this happen then:
All of the spiders are experienced Nen users. When tattooing Hisoka surely the artist would've recognized that they were not tattooing real skin, but rather
Texture Surprise. Since
Texture Surprisewas inspired by a chewing gum wrapper, I wonder if that would even work in the first place, since tattooing paper is probably not very fruitful.