E.g. you can:
- run
gucharmap
- find whatever symbol you need
- double click it
- click on Copy button in the lower right corner
- paste the symbol wherever you want it
For a bit more convenient way:
- press ctrl+shift+U keys
- release U
- enter hexadecimal unicode sequence of the symbol you want while still holding ctrl+shift
- release ctrl+shift
Unicode for @ is 0040 (so you need to type in 40 before releasing ctrl+shift). You can find other unicode sequences in gucharmap.
You might want to tinker with System Settings -> Region & Language -> Layouts, you can choose desired layout there and configure it's options (Options button in lower right corner of the window), however, I couldn't find anything related to @ - maybe it's layout specific.