When you buy components for a project, do you try to use a single or multiple sources?
One source for the little stuff. Best source(s) for the rest.

Do you try to buy from companies in your country or it doesn't matter if it is in another country (import costs)?
In the US I see no reason to buy outside - especially since when buying abroad, you deal wit totally unknowns and may end up with knock-offs.

I heard that some clients preffer cheap components vs quality components, what can you do if this situation arrives?

Cheap components electronic not quality ??? I really can not visualize an electronic component of "lesser quality".
Electromechanical, yes. There I would basically let the idiot - if he insist - put his crap in, there is no fall back on the designer. I would, however put my objection on record.

Erik