The source documents are held in the table StageArchivesInformation. I haven't downloaded the source files but I think you will need something like this (saves the source file to the user's temporary folder):
Dim profileKey As String = si.WorkflowClusterPk.ProfileKey.ToString
Dim scenarioKey As Integer = si.WorkflowClusterPk.ScenarioKey
Dim timeKey As Integer = si.WorkflowClusterPk.TimeKey
Dim sql As New Text.StringBuilder
sql.AppendLine("SELECT SourceFileName, SourceFileBytes")
sql.AppendLine("FROM StageArchivesInformation With(NOLOCK)")
sql.AppendLine(String.Format("WHERE Wfk = '{0}'", profileKey))
sql.AppendLine(String.Format("AND Wsk = {0}", scenarioKey))
sql.AppendLine(String.Format("AND Wtk = {0}", timeKey.ToString))
Dim SourceFileName As String = String.Empty
Dim fileBytes As Byte()
Using dbConnApp As DbConnInfo = BRApi.Database.CreateApplicationDbConnInfo(si)
Using dt As DataTable = BRApi.Database.ExecuteSql(dbConnApp, sql.ToString, False)
If (dt IsNot Nothing) AndAlso (dt.Rows.Count > 0) Then
For Each dr As DataRow In dt.Rows
SourceFileName = dr("SourceFileName")
fileBytes = dr("SourceFileBytes")
Next
End If
End Using
End Using
BRApi.Utilities.SaveFileBytesToUserTempFolder(si, si.UserName, SourceFileName, fileBytes)