Most of the time they will gradually clump up so a single bulb or two will become a cluster in a few years.
These little iris are actually several different species, the most common being Iris reticulata. In catalogs and books they are sometimes called rock garden iris or bulb iris. The blue one pictured above is 'Joyce'. There is also a white called 'Natasha', and a pale blue one called 'Cantab'. I had a dark purple one at my old house which was really striking but I can't remember the name. A second species you often see is Iris danfordiae, the only one I know of that is yellow. For me it multiplies more slowly and is earlier than the others. It is a really bright flourescent yellow with green markings.
You can find these bulbs in the fall at Downtown Home and Garden in Ann Arbor or Coleman's Farm Market in Ypsilanti. For better selection and prices, however, you need to mail order them. I have had good luck ordering these and other bulbs from John Scheepers. As bulbs go, these iris are not very expensive. Last year you could get 100 of most kinds for between $15-$20, about the same price as crocus.
Because they multiply, I like to plant the bulbs singly 3-4 inches apart, in drifts. They take up almost no room and you can even plant them on top of other bulbs like daffodils.