If your chain needs replacing or you'd just like an extra one I sell the chain I use for my necklaces in 3 different lengths. It's a delicate sterling silver chain, each link is about 2mm in length. It comes with the clasp attached and an extra jump ring to add your pendant too.
To attach your pendant open the jump ring by twisting the two ends away from each other as shown in the last image, this will ensure the jump ring keeps it shape. It's best to do this with 2 pairs of small pliers but if you don't have any then tweezers will do the job.