Yes and no. If you have missed a spell that is located in a zone that still existences you can return at any time that zone is available to you and collect the spell. If you missed a spell in a zone that is no longer available then the spell must be bought from the Auction House in Saman. The above covers every spell in the game except Shieldus and Barricadus. Both of those spells can only be purchased by vendors that exists in the game temporary. So it's rather important to buy these spells from Damiroh and Kessle when you have the chance.