It makes no sense to put the combobox items into a text file, especially since you can access which items are in the combobox immediately, and add or remove items. Create a StringBuilder and then loop through the items. My example below assumes textbox1.text contains the string you are looking for.

Code:
        Dim builder As New System.Text.StringBuilder

        For Each comboItem In ComboBox1.Items
            If comboItem = TextBox1.Text Then
                builder.AppendLine(comboItem)
            End If
        Next comboItem

        MsgBox(builder.ToString)
http://www.dotnetperls.com/stringbuilder-vbnet