I know that Comp and Crane Cams will sell you 1 rocker.
Say you have a Comp rocker and the part number for a set is 1032-16 (16 Means 16 Rockers) Take that part number and change it to a 1032-1, and you'll get one rocker.
Before you do anything though you really have to find out what brand they are.
What Color are they????