Refer to this site
link posted below

Here is a link that might be useful: Acer-Palmatum-seeds

My most reliable source is an Acer palmatum tree. Look for seeds on the trees in summer. Collect them as the seeds begin to turn brown in late summer/early fall.

Although this is a pretty old posting from December of last year, I do have a prolific standard acer palmatum which annually produces volumes of seeds. It is not one of the thin leaf varieties, but a simple green leaf. (I have 4 adult trees in my yard with an annual raft of volunteers.) Would be glad to send some of last years seeds or this years in the fall. Contact me offline to discuss any further interest.