I recall from some time I spent in Missouri horse country,
that I went to the county assey office and asked to see the property line records
so I could find out who owned some creek land I wanted to hunt.
Now, this of course was out in the sticks, so the two ladies in the tax office were both helping me find some property, even got some small town gossip on who's mad at who and wont help folks that are their friends. Eventually I left with a list of names & addresses to contact.

You might also want to cruise the net for aerial images of existing club fields,
ones that are very unobtrusive to the original land,
to show potential site owners how little we disrupt his land.
Maybe even get a list of places where the overfly land stays in production for crop or graze.

Also, dont sell short the 'Hey Lookit That' factor of our models.
If you drive out to some podunk town in the sticks some day,
and assemble a 30" foamie and a 90"Extra in the parking lot of the towns only gas station,
I bet you will get the opportunity to call over plenty of locals you catch lookin at the planes
... and while showing them the planes you mention your national organization club is looking to lease some overfly land.
heck, bring some hotshot kid with a 14" micro 3Dfoamie and put on a show in the parking lot to get folks interested