Symbols are normally accessible to anyone who is attuned to them (i.e. introduced to them) but there are times that one's Reiki Guides attune you to a much needed symbol; for example, when one channels a symbol, it's considered that you are automatically attuned to it and that you don't need it to be passed to you via a lineage (otherwise how would the symbols of Usui or Karuna Reiki have been passed along?). There's two ways of getting guided attunements: the spontaneous way where a Guide or a channeling experience attunes you to the symbol, and the request method, in which you ask your guides to attune you, assuming they know the symbol(s) in question.
There is one Reiki Guided-Attunement Meditation on the web by Profiles/TheCrisses which still needs to be modified to include more symbols. An audio tape or MP3 of the attunement may eventually be made available.