(ritual caster feat) how could you gain a ritual book in a Role-Play way?

after a while i decided that i wanted to add the Wizard ritual caster feat to my PC. i talked about this with my DM and they pointed out that i would need to acquire the book someway for this to start. i suggested that my PC could buy one but they noted that doing so would open the option of haveing other PC’s buying simular feats

how have other players gone about this before