I'll quote the text from the Sigil magic page to answer this:
"Characters can learn a number of magic sigils of any type equal to their PER. At first, learning a core sigil (first column) lets them cast a single spell (second column) even if they don’t know any expression sigil (first row) yet. As they progress, they can combine core and expression sigils to cast various spells, further modified by enhancement sigils."
Now, how they get their sigils is decided by the Referee. Are they found, do they research them... you decide.