Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
Dim CliDV As DataView
CliDV = DataGridView1.DataSource
CliDV.RowFilter = "[Ragione Sociale] LIKE '*" + TextBox1.Text + "*'"
End Sub
nello secifico come hai detto te lo da su:
CliDV = DataGridView1.DataSource nella sezione sopra citata