It's possible that you'll need the OpenCV bindings too, not only the libopencv_java.so (never tried, I don't use Java). The best solution would be to check the older package repos for older versions of your distro.
For example, for Debian (Jessie) you can get the 2.4.9 Java pachages (ARM64 platform) from here: https://packages.debian.org/jessie/li... and https://packages.debian.org/jessie/am...
You can also compile it yourself, using the 2.4 branch from Github: https://github.com/opencv/opencv/tree...
Just follow this guide: https://docs.opencv.org/trunk/d7/d9f/...
If it helps, you can give the $30 to a charity of your choice :-))