05/02/01 Email this story to a friend XAVIER ZARAGOZA The Daily Dispatch A Douglas prison in-mate was arrested for purchasing $50 worth of marijuana from an un-dercover cop Tuesday. Oscar Valenzuela, 28, was arrested at Spear Park on Third Street for attempting to purchase an ounce of marijuana, said Mark Wilkinson, public information officer for the Douglas Police Department. Douglas police received information about an inmate who was interested in buying mari-juana, Wilkinson said.