Learning Wizard Spells
The Wizard’s Spellbook
Wizard characters can copy new spells into their spellbook beyond the ones automatically granted at level up. This does not affect their prepared spell limit or spell slots. It only increases the pool of spells they can choose from.
Copying a Spell into the Book
When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.
Copying that spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation.
For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.
Source: 5th Edition SRD
Where to Find Spells to Copy
You can find spell scrolls during adventures. Low level spell scrolls are also available for purchase at the Guild Shoppe (the cost of purchasing the scroll does not lessen the 50gp/level requirement in the rules).
If you come across a spellbook during an adventure, you can copy any wizard spells inside to your spellbook as well.
It is possible to find a library with magical texts during your adventures that could provide spells you could add to your spellbook. The success of this method will vary by DM.
THe guild also has a library, and in downtime between missions you can research spells there. As usual, you would still need to spend the appropriate amount of gold to copy it to your spellbook.