www.freewarefiles.com has a section for screensavers. I can`t say I've really used any of them, but I've used the rest of the site for plenty of stuff and for the most part everything is clean there. Obviously still a good idea to virus scan anything you download.
One question though. Other than aesthetics, why do you really need a screensaver? Unless you have a CRT and never let it go to standby, there's little to no risk of burn-in, and it actually shortens the life of LCD's because it means the backlight is on the entire time, using up it's available lifetime.