I recently flashed my old router with DD-WRT just to try it out again. I did this without backing up the original firmware, thinking that Linksys has it available for download. But it wasn't hosted by Linksys. So then I tried live chat support. They gave me this reason:
Due to the GPL (General Purpose License) rules that govern the use of this firmware, we are no longer able to provide this firmware for download.
It turns out a handful of routers are no longer supported by Linksys.
I found the firmware by asking through the DD-WRT forums. You can find it here: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=154751
If you don't feel like registering, you can also download the firmware through this link: https://rapidshare.com/files/189707480/WRH54G_v1.01.04_029_US_code.bin