My CentOs machine isn't up just yet, but from memory, there didn't seem to be an rpm for amavisd and perhaps not Webmin either.
Building from source isn't hard. Download the package to /usr/source (you can do this by cd'ing to /usr/src then wget the tarzip (ie wget http://something)
Untar the package with tar -xvzf packagename
cd into new package directory.
read the README file and any other interesting looking test files, but typical install procedure is to type
and then configure the package. You may find you need to copy the basic config file to /etc and a suitable startup script (hopefully supplied in the package) to /etc/rc.d/init.d and use chkconfig --add to add the startup file to chkconfig from which you can then turn in on.
That should be about it.