Kristine,

Unless you need a standard that is quite unusual, I highly suggest simply asking other labs for extra well-characterized material that will suit your needs. And that's just if you aren't interested in purchasing standards like Peach Leaf. I think most labs would be willing to share, especially if you can share some data! The extra confidence from getting isotope values that are reproduced in other labs far outweighs most benefits from having your own material.