These days, identifying the brand name is most of the battle. Gerber has been around for a while. Chances are if you take out the hot stem, your local hardware store or plumbing supply will have it. I don't see enough Gerbers to tell you just from the outside which stem it is, but one of the other guys might.
Plan on replacing both stems, and both inside seats. It is a little metal fitting that unscrews with a "seat wrench" which you can get at any hardware store for $10. If you don't change it, odds are it is roughed up a little and will eat the new rubber washers in short order. Not hard or expensive to replace it.