S&W might have a replacement available. Looks like e-gunparts.com has a
rear sight leaf available, but that's just the leaf; you'd have to rebuild the whole rear sight (including breaking the windage nut/screw to get the sight blade out, and then installing and staking a new nut/screw into the new leaf). Unfortunately, the
complete assembly appears to be sold out.
GunBroker.com has a complete assembly listed (auction # 151395270; can't link to a live auction), though I don't know if it'll fit your 686 (namely, the front screw might not be in the right place).