I haven't seen a guide for this that explains it good enough, so here we go.

1. Go to a space station and buy emergency cartographic charts.

2. Go into space and activate chart. If it reveals anything other than "Distress Beacon" don't go to it. The charts will only find one building of each type at a time, so leaving an icon for each type will ensure you won't find any more of those types.

3. If it reveals a distress beacon, go to it and get just close enough to see if there is an npc pilot. If there is, leave. If there isn't, congrats you've found a claimable ship.

Leaving the distress beacon icon for the npc ship will guarantee the rest of the distress beacons will be claimable ships. So now that have all 4 icons you will only be able to find claimable ships with emergency cartographic data.

It seems that your current location within the system determines what the planetary chart will find(you'll get a lot of "building of similar type already charted"), so pulse around repeatedly attempting to plot the data until it works.

Using this method you will find ship after ship, and can claim dozens within a short time. Return to the space station, salvage, rinse and repeat. Happy hunting!