well you can always go to your local Walmart or craft store and buy a ball of yarn the color you need and cut it the length of the latch hook yarn
:-)

Or you can contact the company directly and have them send you the yarn you need since they sold the kit without enough yarn they should send you some thats what i did when i ran out of yarn once

Walmart and Hobby Lobby, possibly Micheal's now sell the precut yarn for hooking.