When purchasing plots, it is not mandatory to immediately buy from the cemetery.  Often times individuals own plots that they are looking to sell for various reasons, and it may be most cost effective to purchase directly from them.  Usually the local Jewish newspapers in Maryland, DC, and Virginia will have advertisements in the classified section, so that is a good place to start with.  Most executive directors at their respective congregations will know of the person who handles their specific cemetery, and if any plots are available to members.  If that doesn’t work out, the cemetery will be able to provide you with what you require.