You can purchase ground control targets from many survey supplies (e.g. Iron Cross Aerial Targets for $15 to $45 a piece).
Alternatively, you can go cheap and make your own. For our Aerial Photography Targets we use a rubber pond liner material material called EPDM Pond Liner, which you can cut down to size. It can be purchased at Home Depot or Lowes in the Gardening. It is a on a roll and can be purchased by the linear foot. It expands to 15 foot width. It is cut it into one meter squares and then painted using a rubberized white paint.
We typically cut ours into 1 m x 1 m sqaures or 0.5 m x 0.5 m squares (depending on flight height) and then label them with numbers that can be seen from sky.
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