Create bacpac from SQL Server 2008
You can use Sql Database Migration Wizard or command line tool sqlpackage.exe for creating .bacpac file for SQL Server 2008.

Download + documentation for SQL Database Migration wizard:

https://sqlazuremw.codeplex.com

This is from Migration Wizard page, which mentions about supporting SQL Server 2008 version. Though in the download links it only starts from SQL Server 2008 R2 but I would give that a try. I have not tested anything below SQL Server 2012.


  SQL Database Migration Wizard (SQLAzureMW) is designed to help you
  migrate your SQL Server 2005/2008/2012/2014 databases to Azure SQL
  Database.


How to use sqlpackage.exe?

https://msdn.microsoft.com/en-us/library/hh550080%28v=vs.103%29.aspx?f=255&amp;MSPPError=-2147217396

There is a step by step guide about using these tools.