Finding modern house design can be a bit challenging because of the loose definition of the term, but after a few initial searches you should get a feel for which sites have better offerings than others. Keep in mind that a site with the largest number of plans is not necessarily the best resource.
There are specialist designers focusing on modern plans, who may offer you more interesting and individual designs and smarter service. It just requires a bit more digging to reach the smaller studios. That said, looking through a large site's collection of house plans can be a quick and efficient way to refine your ideas for what you seek in a modern plan.