Check to see if they offer a premium-grade pine lumber, which many stores do now. It costs more, but you're not using that much for stringers. Also, you can use poplar, as long as you seal it as well as you can and allow the boat to dry well between uses. It is generally very clear and straight-grained wood, but it has low rot resistance. You could also use maple or oak, but it will add some weight unless you use thinner stringers.