While I must concede
@bonsaichile's points, it could be educational as well as a bit of fun. I imagine one of the things that make aspen difficult to collect it that they sprout new shoots from the roots. In a landscape setting, there is no such thing as one aspen or, an entire forest is really just one aspen! Hmmm, what about trying to gather some root cuttings too,
@Warpig?
Regarding leaf size, aspen have a nasty habit of making huge leaves on new growth, especially apical new growth. Leaves on year-plus wood are smaller and smaller lower down the stem/trunk. I have a trio of Himalayan birch in my yard that have similar habits with regard to leaf size and dropping branches. Nevertheless, I am having a ball with seedlings from the trio. Kept small, the leaves stay very small making them entertaining mini-bonsai. Maybe look for some volunteer seedlings too
@Warpig.
Lastly,
@Warpig, do see how it goes with air layering next spring. Maybe gets some mid-spring cuttings too and see if you can get them to root. It is almost certain to be a waste of time if you are envisioning making prize winning bonsai. However, I find following such larks to be highly educational/entertaining, so just ignore
@bonsaichile's realistic discouragement and go do it.
All work and no play makes for a really dull hobby.