Marketing to the suppliers - tools of engagement

I've been playing with the idea for some time now, actually trying to build my unfinished master thesis around the subject. I have numerous discussions with different kind of IT suppliers on daily basis. All of them are interested to provide their services to the clients i represent as MSP provider. Issue is that the need on clients side is much more narrow than the array of the services offered. The sales personnel get frustrated of no new business opportunities, as they provide the services in wrong categories, and the suppliers already inside the organisation sell additional services that client could have procured from the market for better rate. So it would be beneficial to tell your potential suppliers on what there is to offer, and not just react on what the stakeholders bring to sourcing to sign off. Do You market your needs to the suppliers, or let them dig them out for You, so their solution would see to be best fit?