Take a look at the next sentence after the one you quote (from the same the sidebar in the Wizard section):
For each level of the spell, the process [of copying a new spell into your spellbook] 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.
This would suggest that you find some merchant somewhere who sells the requisite inks, paper, and miscellaneous reagents to copy the spell.
It is important to note that Wizards can learn learn spells both when they level up and by copying spells they find during their day-to-day adventures (for instance, if the wizard comes across a spell scroll that holds a wizard spell s/he does not yet know).
The cost of copying a spell does not apply to the two spells you get each time you level.
Each time you gain a wizard level, you can add two wizard spells of your choice to your spellbook for free.
So copying the spell scroll above would require 50gp but if you level up and pick the same spell, you would know it for free.