You can find roms on this site, or at XDA. I don't use rom manager to download and flash. I always use a pc. Rom manager has limited roms, but it depends on what you are looking for.
Type "evo4g roms" in Google and the first one to pop up is andrdcandy..it has a bunch. They link you to xda so you don't have to search all over for them.
I agree. XDA has a good list of roms. I prefer flashing roms from amon_ra recovery, its easier for me. I run CM7 at the moment, has its bugs but its a good rom.