When adding a stock item that doesn't have a barcode specified, you can generate your own barcodes through Retailer.
- From the Stock Screen or the Find Stock Screen, click on Add.
- On the keyboard, press [Alt key] + [G].

Click HERE for more information on adding a new stock item.

If you need to create your own barcodes for stock in bulk, you can use the Import Supplier Stock File utility along with an Excel spreadsheet.