I don't know much about tattoos, but you should probably do some research on the internet beforehand, and make sure the tattoo shop you're going to is legit (i.e. properly cleans/sterilizes their instruments after each use).
I've wanted an "A" necklace like that for some time, too, but I've never been able to find one. Aside from any "A" jewelry we've seen Ayu herself wear, I don't know if they make any!
