I was able to resolve my own problem perfectly. I used the customvalidator OnServerValidate Event
Now in my TextChanged event I show up a warning if the data entered is not correct and in the button_click event of my submit button I call Page.Validate() that subsequently calls OnServerValidate event handler of each custom validator associated with a text box.
protected void btnIssueItem_Click(object sender, EventArgs e)
{
Page.Validate();
if (!Page.IsValid)
return;
....
}
protected void tbRoomID_CustomValidator_ServerValidate(object source, ServerValidateEventArgs args)
{
BAL bal = new BAL();
args.IsValid = bal.GetRoomByRoomID(Int32.Parse(args.Value)).Count == 0 ? false : true;
}