How did you find a mentor when nobody at your organisation does reward?

I look after compensation and benefits for a 260-person architecture and engineering firm in Vancouver. I report to the chief operating officer, who is supportive but has never worked in HR, and there is nobody else in the building I can ask whether my approach is sensible.

I am four years in and mostly self-taught. I have reached the point where I do not know what I do not know.

I have seen the mentoring section here but I am unsure about approaching a stranger. For those who have a mentor or are one: how did it start, how often do you speak, and what makes someone easy to say yes to?