Suitable rocks are not that easy to find, as most that you DO find can't be climbed, and mentioned. I've had some luck with rivers (one character had the luck of having one suitable rock 4 meters from the homestead). However, I've also had a character who spent a lot of time first walking around a mid sized lake, and then punting along the shore of a huge lake to eventually find a suitable rock.
I think the advice to keep the eyes open for them from the start is a very good one, as is the marking of fords.