When I've used all of the type of beads I have for a necklace, I now add a simple piece of chain on each side of the necklace to extend it. While I usually make my chain, it can certainly be purchased. I simply attach the strung beads to the pieces of chain and then put the clasp on the chain. I'm usually only a few beads short and the chain doesn't show in the front of the necklace. By the way, I like to use this method for double strand necklaces as well.

Since I do sell some of my work, I actually like to add a few links of chain to many of my necklaces. When I add an ample amount of chain to each side of a necklace, it's easy to shorten a piece for a customer by removing links. If I need to make the necklace longer, I can make and add more links. I generally carry extra links or extenders (see: https://www.wirejewelry.com/jewelry_making_tips_techniques/Easy-Necklace-Extenders_178.html) with me. This also makes it easy for me to alter the length of my own necklaces for wearing with different garments.