I personally use Getty Images ’cause they have the best footage and the best web based search systems. Besides they own everyone else!
when you talk to the stock house(s), just tell them how the video will be used, how often and how it will NOT be used. They have different usage rates for broadcast,non-broadcast and one time exhibitions. And you may get a little price break depending on how many clips you purchase. And don’t be surprised if they ask for a copy of the finished video as well! Just be up front with them.