There are many types of yard hydrants each with slightly different components. Each hydrant brand offers a hydrant repair kit for their product. To begin a yard hydrant repair it is important to get the repair kit for the hydrant brand that you have.
The brand name should be on the top of the hydrant. Call your local home improvement store, hardware store, and/or plumbing supply house and find out who carries repair parts for the yard hydrant you have. Sometimes this may require ordering the part online.