Cross Country and many other large companies are in Florida. In the late '90s, CC, then the largest travel company merged with Travcorps, the second largest agency - which was based in the Boston area and still maintains offices there. Other east coast agencies that come to mind include Millenial, PPR, Trinity, and Sagent. A number of the brands now largest agency, American Mobile, originated on the east coast and may still have offices there.
Anyway, as dance4life implies, it doesn't usually make that much difference where an agency is located.
As far as sites like Indeed are concerned, it would be entirely coincidental if an assignment you see there is current and available. Those jobs are "representative" at best. At worst, they are outright lies!