  Internal DLA (IDLA) is an internal aggregation model in which particles
perform random walks from the origin, in turn, and stop upon reaching an
unoccupied site. Levine and Peres showed that, when particles start instead
from fixed multiple-point distributions, the modified IDLA processes have
deterministic scaling limits related to a certain obstacle problem. In this
paper, we investigate the convergence rate of this "extended source" IDLA in
the plane to its scaling limit. We show that, if $\delta$ is the lattice size,
fluctuations of the IDLA occupied set are at most of order $\delta^{3/5}$ from
its scaling limit, with probability at least $1-e^{-1/\delta^{2/5}}$.
