As part of Project Euler I have to find the largest palindrome made from the product of two 3-digit numbers.

I have created this code but when i3 gets to 909090 there is an error which states largest "Index was outside the bounds of the array." on the line
Code:
If i3(0) = i3(5) And i3(1) = i3(4) And i3(2) = i3(3) Then
Here is my full code, I'm not sure exactly what is wrong?
Also, i3 is a string because I couldn't work out how to go through an integer digit by digit?

Code:
Public Class Form1

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        Dim i As Integer = 999
        Dim i2 As Integer = 999
        Dim i3 As String
        Dim found As Boolean = False

        Do Until found = True
            If i2 > 1 Then
                i2 = i2 - 1
            Else : i2 = 999
                i = i - 1
            End If

            i3 = i * i2
            If i3.Length = 5 Then
                If i3(0) = i3(5) And i3(1) = i3(4) And i3(2) = i3(3) Then
                    found = True
                End If
            ElseIf i3.Length = 4 Then
                If i3(0) = i3(4) And i3(1) = i3(3) Then
                    found = True
                End If
            End If
        Loop

        MsgBox(i3)

    End Sub
End Class