Fresh water pump hook up issue

Just finished installing a new water pump (Shurflo). Found that it ran but did not pump water. Traced the problem the compression fitting on the input not being tight so there was a vaccumn leak. I screwed the hose fitting as far as it would go, but no good. Seems that there are not enough threads on the filter side to allow a tight fit. I remember seeing a flat washer on the ground when I took out the old one and thought nothing of it. I'm guessing that it went in to the fitting to make a tight fit. The pump and filter seem to be standard regardless of brand. How do I make a tight fit or where do I buy the washer.