Espresso is extremely sensitive to the quality and freshness of the coffee - more so than with any other method of coffee preparation. When preparing espresso, coffee noticeably loses flavor within a week of roasting its beans and literally within minutes of grinding them. (The smell of coffee is also unfortunately the smell of its flavor escaping.) Because coffee quality can vary significantly between suppliers, the choice of a supplier can have a big impact on the espresso.
For example, when it comes to espresso, Illy coffee is generally held in high regard. (So much so that I've caught at least a few establishments claiming that they served Illy when they clearly did not.) Yet despite Illy's impressive quality controls and vacuum sealing, their coffee still arrives from Italy several weeks after roasting. While Illy beans produce a very consistent and high-quality cup, I've often found better results when a local coffee roaster is used with beans roasted just that week.
Some of the better local roasters include Mr. Espresso, Graffeo, and Caffé Roma. At the other end of the spectrum, I rarely found a decent espresso produced with America's Best Coffee, Peerless, or Superior coffee. However, this may say less about the quality of the supplier than the tendency for lower-quality espresso producers to use them.