As previously mentioned, spellbooks are great loot. Wizards (mostly) will have a small spellbook with them while traveling and a more complete book (or series of them) at their base of operations. Some may keep their whole library in a bag of holding, others might use a demi plane, but generally speaking, a wizard probably isn't carrying his journals for the last several decades on him, and that is what a spellbook is. These traveling spellbooks are a great way to include some useful spells that players may not learn on their own.
Scrolls are your other solid option. Scrolls can be loot from any creature capable of using them, but they can also be found discarded by those who don't value magic or writing. I've even had a spell scroll with a map sketched on the back. Many scrolls might be found in a wizard's lair, but they could also be tucked on a dusty shelf in a rare book store.
Other options do exist. You could find a spellbook buried with its author. One that was passed down through muggle relatives might be given as a quest reward. A crafty wizard might also use frescoes or hieroglyphics in a dungeon as a permanent spellbook. Spells can even be inscribed on objects like a shield or staff. One could even find that a wizard had his spellbook tattooed on his body. Some of these have obvious limitations, but a scribe wizard is well suited to quickly copy a spell or two while the rest of the PCs take a short rest.
Edit: I forgot the obvious ones. Swapping spells with another wizard, and buying spell scrolls to copy from. Depending on the campaign, these may not be easy to find, but finding a scroll (or spellbook) for sale now and again isn't out of the question, and wizards that are friendly may be willing to trade if approached correctly.
