Suffix Numbers With A Through To IV

I have a macro that will add the next letter to a number as a suffix but this will only work for A - Z. But i need variations of upto 150, so i need the macro to add another letter to the variation once Z has been reached. For example at the moment 1000A the next variation would be 1000B up until 1000Z but once this has been reached the macro then starts to use symbols as the code is written using ASCII numbers. So i need the next variation to be 1000AA then 1000AB etc the code i currently have is


Public Function NewVarianceLetter(lngQuoteNum As Long, Optional rngQuote As Range) As String
Dim a As String
Dim i As Long
Dim rngFind As Range