Yes, it is in Celestia. It's kind of inter-dispersed throughout the whole world. I believe you can Train Gobbler, Keen and Strong straight away from the trainers as long as you are level 50. There are more trainers available with more powerful spells as you continue to progress through the world. Hope that helps, and Good Luck!
You can get a basic selection of astral school spells as soon as you enter Celestia. Located on the right side of the Celestial Base Camp. You will find more advanced astral spells as you progress through the world and beyond.
Once you have access to Celestia, you can immediately go to the Astral Trainers and get some new spells. However, there are also additional astral trainers scattered around Celestia with more powerful upgrades.
You get the astral spells every 2 levels. Some are in Celestial Base Camp, other spells in a dungeon the Stellaruim. For example, you enter Celestial in level 50, you get another at level 52, 54, 56, and so on.