You have :put
:put=&spellfile
You have i_CTRL-R_=
blabla in insertmode ^R=&spellfile^M
(with ^R being CTRL-R typed in insert-mode, and ^M the carriage return you'll type to validate the input given to CTRL-R=)
If you want to put it into a register -> :let @a = &spellfile
(or any other register name -> :h registers
)