Welcome to the VRCC and to owning the greatest motorcycle ever made!
Yes, rear speakers were an option on the I/S. You can purchase a new set for the rears from a few different vendors:
OEM Speakers:
http://www.directlineparts.com/product.asp?pid=290&str=2J&M used to make a set for the Valkyrie, but I can't find them right now.
Polk DB501 will fit with some modifications:
http://www.polkaudio.com/caraudio/products/speakers/5/db501/ If you do a search on this site for "polk speakers" or something of the sort, you should get a link to the modifications needed to install them. It involves some trimming of the speaker ring and the speaker pods, but it is nothing major. I changed both front and rear with the DB501's... supposedly the sound is a ton better... I can't say it is a huge difference, but it is better.
The connection for the rear speaker wires is under the right side cover... it is a white connector with four holes (2x2).
An aftermarket supplier for the speaker wire harness is
http://www.electricalconnection.com/. I bought mine from him it is quality... the only thing is that it has gray sheathing so if you try to run the wire external of the trunk like Honda did it would be too visible, I did an underneath - interior run of the wire; involves some drilling but not too difficult.
** edit - sorry for the repeat information, was typing while the others were.