Radars can be bought in Tamer's Paradise (after the ending boss). The cheapest cost 40 feathers and the most expensive 1000.
You'll have more than enough feathers just by finish kudos along the main story !
Place yourself in a place where you are sure to find the Tem of your choice and activate the radar.
Be sure to have a mount cause you only want to fight the Tem you choose or you'll be breake the radar.