motherboards usually come with oodles (a technical term, as <a href="http://www.dansdata.com">dan</a> would say) of extra screws and standoffs etc. Try to find someone who's assembled a computer or two in the last few months or try the repairs dept of (insert name of local big-name computer dealer*)... they should have.
* sorry i'm not American, so don't know the names... :)
or use a washer and some superglue... or something ;)
Slave to the Bar.