Where else can I download the legacy Java SE 6 runtime?
Go to https://www.apple.com/support/ then search for DL1572

See Apple Knowledgebase pages often blank

 EDIT - or in this case, not… as it doesn't seem to fix it

Update:
Original KB link appears to be working again.
Would appear to have been an Apple issue, now resolved.
https://support.apple.com/kb/DL1572