So I seem to not have my Snow Leopard DVD available anywhere.
I can't use a newer version like Lion.
What can I do to repair/recover Snow Leopard now that the installation DVD is not available?
If you computer not booting is the reason you need your Snow Leopard disc, there are several things to try and troubleshoot your issue.
An unrepairable disk error requires an Erase/Install of OS X. It may not cause issues right away, so get the files transferred now. In fact it may fail to even boot up. The previously mentioned shift key at boot (Safe Mode) worked.
I just used Disk Utility to BACK UP the entire hard drive to a properly setup USB. That will repair the installed OS X 10.6.8. Carbon Copy Cloner allows bulky user files to be excluded. I made a copy of OS X 10.6.8 installation right after Software Update was finished. Thanks to apple discussions for tip on using a copy to repair the system.