Remove the quote in front of each function symbol. You've already quoted the alist.
Just look up the choice in the alist to get the function, then call it.
(defun find/buffer ()
(interactive)
(let* ((choices '(("find-file" . find-file)
("switch-to-buffer" . switch-to-buffer)))
(choice (completing-read "Choose one: " choices)))
(call-interactively (cdr (assoc choice choices)))))
But if you make no difference between the name to choose and the function-symbol name then you might as well just do this:
(defun find/buffer ()
(interactive)
(call-interactively
(intern (completing-read "Choose one: " '(find-file switch-to-buffer)))))