You can use purchased chain for extenders, or make your own.
I usually make mine because it's easy to adjust the length just remove a link or two.
I keep a supply of extenders at the boutiques that sell my jewelry and bring them with me to art shows.
These little pieces have saved many sales.
Some of my regular customers count on me to adjust their necklaces with extenders.
If a customer prefers not to use one, I can use the chain from an extender to permanently lengthen the necklace.
It's a simple, flexible solution that works great.
The extenders shown in the photo are copper, but I always keep both copper and sterling silver on hand.
Be ready for customer preferences they matter.