Hello,
JUNOS 7.6 has been End-Of-Life since 15 Aug 2007 meaning it is retired and You cannot download it from official website.
Regarding JUNOS 7.6->9.3 upgrade experience for J2300: I don't have any but maybe other posters can chime in.
General notes:
1/ the direct upgrade is supported across 3 or less major releases. This means if you are running 7.5, then the supported upgrade path looks like 7.5->8.1->8.4->9.1->9.3. However, you can attempt straight 7.5->9.3 upgrade, preferably in the lab, to see how it goes.
2/ please study release notes for JUNOS 9.3, especially regarding compact flash and RAM size for J2300.
3/ please open a JTAC case to obtain assistance if something looks unclear/you have special requirements
HTH
Thanks
Alex