KDE Look have a section devoted to them, but there are only a handful. We expect that to change as KDE4 becomes more widely used.
A search on Google reveals nothing really useful either.
Now there is an answer that I really hope will catch on and work out. Some kind soul over at the Gentoo forums have compiled a plasmoid overlay for us Gentoo folk.
To quote the master:
There have been several questions, what the best way to install extra plasmoids for KDE4 is on IRC and in the KDE4 threads and therefore I decided to create an overlay that holds a collection of plasmoids to let portage install them.
In principal you find a site with some good plasmoids, you add it to the overlay (how to do that is explained in the post and asking there will get you an answer real quick.) This way we stay up to date with all plasmoids we can find on the web until they become more popular.
Simple, yea if you are using Gentoo! Anyway, head on over there, get the overlay and help spread the plasmoid goodness!