Pekins are just one of many breeds of domestic duck available. Unless you have a particular reason for wanting the Pekin breed, think about what you want in a duck and choose a breed that most closely resembles those characteristics. I need ducks for eggs because someone in my family is...
I don’t have Pekins, so I don’t know much about them. Many hatchery websites have pictures of their various breeds at different ages, so that may be a good resource for identifying the ducklings you want.
Tractor Supply here does not sex their ducks or chicks. They are all straight run, which I have found can mean more males than females, since females may have been pulled out by the hatchery to be sold to vendors who do sex their stock. In addition, TSC here does not segregate their ducklings...