I think you only get it at the end of the game if it is a T-Rex you have chosen in the beginning. Otherwise no way to get it, maybe cheatcode?
You can find a T-Rex in the core of Mount. Krakanak, after beating Hot Spring Heights in Ilium Village.
You can find him at bonemoth
Spinax is donation point vivosaur, meaning its fossils can be bought with donation points.
ANy random dig site just be lucky
you get one as a prize at the cup. There's also a chance to find these when digging. You can also buy these and gold fossils with donation points.
From what I know on the site, it says you can find him at Mt. Krakanak. Check the related link below.
If you meant Saichan, then Mt. Lavaflow in the regular Fossil Fighters. I'm not sure about Fossil Fighters: Champions.
Treasure Lake
Petrified Woods
At Parchment Desert
You can find Salto fossils in the BB Base.
in sea floor cavern
You can find allo in the rainbow canyon