Get yourself a small digital scale that can read in 1gram increments. You will be amazed at the difference in weight between seemingly similar pieces of wood. Remember that the lighter the overall plane is the less strong the overall construction needs to be. Look for wood that has nice straight grain with no soft spots. You can also get information regarding grain/cut types from some balsa suppliers. Especially if you are planning to do foam wings the weight savings can be large if selecting light wood. Don't fool yourself into trying to build a plane that will survive bad landings. Heavier planes have the nasty habit of self-destructing because of the excessive weight. Light planes bounce, heavy ones stay in one spot, re-kitted.

Some balsa suppliers will supply contest grade balsa. Money worth spending.

Safe Flying! & happy building.