Here http://skeiron.org/tablets-dev/maemo_dev_env_downloads/ you will find the flasher utility.
The firmware files are here http://skeiron.org/tablets-dev/nokia_N900/

For CSSU go to the wiki page http://wiki.maemo.org/Community_SSU with your N900's web browser and tap the stable or testing logo. This will download the install file, that opens Hildon Application Manager to add repo and packages.