Which is correct:
- Sources of information include vendors and consultants, each of which usually has an interest in selling something.
- Sources of information include vendors and consultants, each of whom usually have an interest in selling something.
Which is correct:
- Sources of information include vendors and consultants, each of which usually has an interest in selling something.
- Sources of information include vendors and consultants, each of whom usually have an interest in selling something.
Are they people or companies? People take whom and things take which. Companies are things, not people (despite the statement of politicians and legal analysis to the contrary).
If, when talking about your vendors and consultants you are clearly referring to individuals, use whom. Otherwise which.
And as suggest in the comment, use has since each is singular.