Note: There isn't a reason I can think of to use this because excel can accept column numbers, but here is the function regardless.
Finding Last Used Column or Row
Function Col_Letter(lngCol As Long) As String
vArr = Split(Cells(1, lngCol).Address(True, False), "$")
Col_Letter = vArr(0)
Dim lCol As Long
lCol = Cells(1, Columns.Count).End(xlToLeft).Column
MsgBox "Column Letter" & Col_Letter(lCol)