How are you populating your DevExpress GridControl?
Are you using a stored proc or sql query then adding it to a DataSet and binding the table from the dataset to your grid? If yes, your column name should be set automatically based on the binding query.
Going by the error message, I'd say that the column "Supplier No" doesn't exist in your grid. You can either rename the column via the grid control designer or change the line of code that is erroring to the correct column name. Note that the GetRowCellValue call is overloaded so you can pass a text value for the column name or you can pass the actual Column object (as specified within the grid designer for the column).
You also need to watch if you have grouping switched on in your grid and multi-select and the user puts one or more groups in as unless you put code in to stop groups rows being selected, they will be included in your processed selected records if the user does a shift click to select a block of records. Groups rows will have a negative row Id so I suggest you put a test in for those so you can ignore them otherwise this might be the cause of your error if it's not the column naming - see below: -
foreach (int rowHandle in view.GetSelectedRows())
if (rowHandle >= 0)
int cellValue = (int)gridView1.GetRowCellValue(rowHandle , "Supplier No");