I have bought lots of stainless steel chains via a wholesaler here in Sweden (not that I used it for bondageplay but rather for anchoring a boat that my friend used to own but nevertheless)
. Hm yeah the only real viable option for insertion would be stainless steel. If you talk to a spe******t (eeh why does the forum censor s.p.e.c.i.a.l.i.s.t) and no I am not talking about those large chains like Home Depot or Bauhaus/Hornbach here in Sweden but rather the specialized hardwarestores with just a few employees. They normally know what the heck they are talking about and can order just about anything you want.
I just love the smaller shops cause they are much more friendly and helpful to whatever need you might have.