With JDK8u40, according to this:
Starting with Oracle Java SE 8u40, Oracle does not provide a separate set of accompanying JavaFX Scene Builder binaries. If you would like to contribute changes, ideas or just let us know what you have done with the code, please consult the OpenJDK Community contribution guidelines and join the openjfx-dev mailing list.
You can still download the last binary at Oracle in this link. But check this warning:
WARNING: These versions of JavaFX Scene Builder may include components that do not contain the latest security patches and are not recommended for use in production.
This means it doesn't include the last changes of 8u40, like the new Spinner
control.
Alternatives
The project is open source, and you can find the updated code at the OpenJFX repo. So you are free to download it and build it.
Since its released under BSD license, Gluon is offering support for an updated version of Scene Builder, and you can download an installer for your platform or an executable jar from here.
Besides, they have an open repository where anybody can contribute.
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