Take a look at the following sentence after the one you quote (from the same sidebar in the section of the wizard):
For each level of the spell, the process [of copying a new spell into your spellbook] takes 2 hours and costs 50 inches. Cost represents the hardware components you spend as you experiment with the spell to control it, as well as the fine inks you need to record it.
This would suggest that somewhere you find a merchant who sells the inks, paper, and various reagents required to copy the spell.
It is important to note that wizards can learn spells both when they level up and by copying spells they find during their daily adventures (for example, if the wizard falls on a scroll containing a spell of sorcerer, he does not know yet).
The cost of copying a spell does not apply to both spells at each level.
Whenever you win a wizard level, you can add two wizard spells of your choice for free to your spell book.
So copying the spell parchment above would require 50gp, but if you turn up the level and select the same spell, you'll know it for free.