Hi,
search for an local framebuilder. Fat Chance Bikes, as desirable they are, are known to rust through. If you are in Europe, gleiss-rahmenbau.de made several seat tube replacements on Yo Eddys and did an great job replacing the cracked seat tube and rusted downtube on my Yeti FRO. In USA I would ask IF Bikes if they repair Fat City Bikes or maybe Chris Herting of 3D Cycles in Durango, who's also known for doing frame repairs. But after all, most local framebuilder can repair your Yo Eddy and replace the seat tube.
Greetings,
Michael