Media buying is really designed for larger purchases of advertising space. The discounts that are offered can be very profitable. The only issue is that you either need to be big enough to take advantage of these discounts or be part of a group that can access these discounts.
Depending on which media you intend to advertise in, you will need to search out companies that can offer you their services to represent your company. We try to keep up with companies that can help in this area but your own research for local firms will be advisable.