You probably need to contact each company to ask permission to use their logos & they should provide you with one - often with details on corporate format crap i.e. so many mm/in away from text, minimum size & whether another corporate logo is allowed on the same photo/poster.
Hi Michelle, you're probably right on the permission aspect, I did find a coulpe site that have free logos, just not enough of them.
What I am doing is, doing some small posters ( 13x19 ) for the guys in the district, some want to use them to thanks sponsors and supporter, simular to this