Aspen shoot blight
The reason you are not finding any fungicides for Venturia shoot blight on aspen is there aren't any specifically labeled for it. A general fungicide labeled for leaf spots might help, however.
Here are two fact sheets containing some information about the disease and managing it:
Sanitation is key. As leaves and twigs drop, rake them, bag them, and remove them. This will help minimize the problem in succeeding years. You can also carefully prune out dead branches. Selective pruning to improve air movement, particularly in lower areas of the trees, will also help keep humidity down, and may help decrease disease severity.