Seeking source of clipart (free or fee-based) that can be used commercially

Hi folks,

Can anyone recommend a good and preferably extensive source of clipart
that can be used commercially, including on websites and on logos
designed for both myself and for clients? Preferably a flat-fee source
for the use of a library, rather than per-image, but any leads you have
would be appreciated. Thank you!
John Rearden
news (AT) reardenwebdesign (DOT) com