As of the 23rd of December you will be able to buy a SmarTrip card from 190 CVS locations in the DC, MD, and Washington area. The cards will be available for $10 which include $5 which can be used for fares for Bus and Metro Trips as well as Metro parking. You can also purchase the cards at Metro Stations and select Giant Stores.
You can add value to the card at either a fare machine at a Metro station or using the fare box on a Metrobus. I am assuming that you can also add money to you card using the fareboxes on the regional buses that accept SmarTrip.