You can use the SelectedIndexChanged event for the combobox in the VB code behind. Add code in this event to change the textbox.
You will also have to be sure the AutoPostBack property of the combobox is set to True. This way whenever the combobox is changed, the page will postback and the code in the SelectedIndexChanged event will run.
Update: 27 Feb 2013
The AutoPostBack property is not in the VB code. You can find it by clicking on the combobox in the designer and then looking in the properties window. You can also change it directly in the ASP.NET code. Something like this:
<asp:DropDownList ID="SomeComboBox" runat="server" AutoPostBack="True"></asp:DropDownList>
Then you will need code to change the textbox in the SelectedIndexChanged event in the VB code. Something like this:
Private Sub SomeComboBox_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles SomeComboBox.SelectedIndexChanged
If SomeTextBox.Text = "available" Then SomeTextBox.Text = "unavailable"