I do not recommend trying to mail fresh yeast. We are in Manhattan, Kansas and we have difficulties getting several cases of it to us each week from Kansas City. Instead, consider buying a supply of IDY instant dry yeast. Unopened it has a shelf life of a year or more and you should be able to get it from any restaurant supply. You might even be able to talk a local baker into putting in an order for a few extra pounds when they place their yeast order. If you want just small quantities, you local supermarket probably carries the small packets of IDY.
Tom Lehmann/The Dough Doctor