Lowes and Home Depot sell various sizes of o-rings. Just take your old one with you to match it. If you have to run skimmerless for a little while then you might want to increase your water changes. Your sps corals will probably not like not having a skimmer but everything else should be fine for a little while.
Have you checked ebay for used sumps?
http://cgi.ebay.com/150-Gallon-Aquar... QcmdZViewItem
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