What is the feminine form of the name Spencer?

In English there are no masculine or feminine forms. English uses gender specific nouns for male or female.

The name Spencer is historically a name for a male, derived from the Anglo-French espenser and the Old French despensier, a dispenser of provisions, a butler or steward who had control of the provisions of a household, positions held by a male.

Today, Spencer is a name usually given to a male, but the family name Spencer is the name of everyone in the family; for example, Lady Diana Spencer, Princess of Wales.