Plan 9 can be downloaded free of charge from: http://plan9.bell-labs.com/plan9
If you're after nice printed manuals and a pressed CD, see: http://www.vitanuova.com/cgi-bin/order.pl?PRODUCTSET=PLAN9 The listed price is £38, €45.42, or $75.00. You used to be able to get just the CD or just the books, but I didn't see such an option now.
