Just keep checking CL and FB Marketplace daily, something will show up. You can set up a saved search on CL that will alert you to any new posts. I'd do several, "van seat", "E350 seat", etc. I'm no expert but if E150 and E250 seats will work, set up a search on them as well. They're definitely out there since so many are getting rid of them in order to do their conversions. For some reason, the mounts, seat belts (especially shoulder belts) are harder to come by than the seats.
2005 E350 Super Duty Ext Wheelchair Van
2002 Itasca 35U Motorhome