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
. SinceTexture Surprise
was 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.