Have you been checking out the various garden centers and nurseries in your local area? This is a great time to do this. Just before summer sets in, there is a shift in the varieties of plant material available.
There is also a change in the blooming of the perennials as we move along though the seasons. I find that if I check out the perennials when they are in bloom, then I will be sure to get the right colors that I am looking for in specific areas of my gardens. For me locally there is Seawright Gardens in Carlisle MA where they specialize in day lilies and hostas. In a couple of weeks I am going to visit there and pick up several varieties of day lilies and hostas. My problem is limiting the number that I want because they are all so beautiful.
Check out any specialty places that might be local to you. If not, find a knowledgeable garden center or nursery and they can special order plants in for you if you have something specific in mind for your garden.