You can use purchased chain for the extender or make your own lengths. I usually do the latter since I like to be able to quickly change the length of the extender by removing a link or two.
I make extenders and leave them in the boutiques that sell my jewelry. I also take them with me to art shows where I vend. These little jewels have saved many a sale. I have some regular customers who depend on my willingness to lengthen their necklaces in this manner. If the customer doesn't want to use an extender, I can steal the length of chain from one and permanently lengthen the necklace. It works great.
The extenders in the photo are copper; however, I like to keep both copper and sterling silver extenders available for use. Be prepared for customer preferences.