The snapshot jars are stored in svn with the website, in the site tree. You can run svn log on build/site/binaries in the site tree to determine the revisions where snapshots were added or changed:
svn log https://svn.apache.org/repos/asf/db/derby/site/trunk/build/site/binaries/ > binaries.log
You could either then go back to that revision if you have a local copy of the website, or use viewvc to get a download link without having to check out the site tree.
As an example, from the log you can see that 10.2.0.3 was added at revision 413060. Going to the build/site/binaries directory in the site tree with viewvc, and setting the directory revision to 413060, you can see the 10.2.0.3 snapshot:
Clicking on db-derby-snapshot-10.2.0.3-412239.zip takes us to this page:
which contains a download link for 10.2.0.3.