  In this paper we prove the existence of a family of self-similar solutions
for a class of coagulation equations with a constant flux of particles from the
origin. These solutions are expected to describe the longtime asymptotics of
Smoluchowski's coagulation equations with a time independent source of clusters
concentrated in small sizes. The self-similar profiles are shown to be smooth,
provided the coagulation kernel is also smooth. Moreover, the self-similar
profiles are estimated from above and from below by $x^{-(\gamma+3)/2}$ as $x
\to 0$, where $\gamma <1 $ is the homogeneity of the kernel, and are proven to
decay at least exponentially as $x \to \infty$.
