Constructo modifications can be found on planetoids as well as prizes from the arena, or hidden on planets. Most of these are going to be easy to locate, but if you're unable to find them, wait until the end of the game and defeat the Raritanium Tournament (round 4) for the Treasure Map. Not only will this show you the location of Gold Bolts and Zoni when you land on a planet, it will also show you what is left in each of the systems you can access.