They are frequently available at the Asian markets in St Paul. Dragon Star on Minnehaha and Dale has them regularly, as does Shuang Hur at University and Dale. Can't speak to the Minneapolis side of the river.
FYI, they're usually sold green since most folks in SE Asia eat them that way dipped in salt, sugar, and dried chilis. You'll have to paper bag 'em to get them ripe enough to eat soft or juiced.