It is a Price Pfister. I forget the model name. This part is available, but is no so common, because this model was in production for relatively few years.
There are several parts outlets on the internet. Any good local plumbing supplier will have it. In Home Depot, the sku # is 550-070, but not all the stores have it, but they can order it.
Where are you? If you are in San Diego, I can tell you several places that stock this part. Expect to pay between $50 and $60.