Solaris 2.6 ships with BOOTP/DHCP support.
Software for Solaris 2.5/2.5.1 is available from Sun's website.
Click on the license agreement form (after reading it) and download the SUNWdhcsr and SUNWdhcsu packages. You'll also need to install SUNWfns, but that's a standard part of Solaris 2.5 provided only for completeness.