Depends on how you define "hard to find". Your corner LBS won't have them, but then they probably won't have ISO349x35 for your Brompton or ISO406x28 for your Tern Verge X30h either.
Birdys use 18" tires.
Maybe not 406x28, but you can buy 406 tires at Walmart that will fit in a pinch. A larger LBS will have 349 tires since they are used on recumbents. But of course if you plan ahead and order a spare 18" tire this won't be an issue unless you plan to tour with the Curl.