You get astral spells once you unlock celestia. There are little stands/trainers in the base camp( the commons of celestia). As you progress and unlock more areas you'll stumble upon the same looking stands but with more spells. You must be a certain level to train certain astral spells. Good luck in your journey!
You learn Astral Spells in Celestia once at level 50. There are 3 schools (as you may know) Moon, Sun, and Star, and just an FYI Sun is the best school to train. There are also 3 dungeons that were originally the schools of Astral Magic which have trainers that have higher level spells than the trainers in Celestia Base Camp (excluding Sun Magic that has its trainer in the Floating Lands). Examples are Infallible for Star in the Stellarium, different Polymorphs (I don't train Moon so I don't know the polymorphs), and Unstoppable and Gargantuan for Sun. Hope this helps Megan Frostflame Level 95