There are a few options for purchasing barcode labels. The easiest is to purchase online.
Barcode labels are pretty standardized with LTO media. The most typical barcodes are 6 digits in length. Most people start with barcode 000000 and work their way up from there. In addition, the label typically has 2 digits at the end as a helpful way of seeing the generation of the media. For example, you may see a label with 000000L6. The “L6” portion of the label means that tape is an LTO-6 tape.
There is special consideration for cleaning cartridges. The convention for cleaner cartridges is to use the letters CLN for the first 3 digits on the label. For example, you may see a label for a cleaning cartridge that is CLN000L1
The human readable text for the label can be oriented horizontally or vertically. I find with autoloaders that vertical is best. When the tapes are stored in a clear case on end the label is easier to read.
There are a few options for purchasing barcode labels. The easiest is to purchase online. Note: some resellers will bundle free labels with bulk tape media purchase.
Here are 2 links for LTO-5 or LTO-6 sheets with 20 labels per sheet from an online backup centric reseller BackupWorks.com.
http://www.backupworks.com/LTO-5-barcode-labels.aspx
http://www.backupworks.com/LTO-6-barcode-labels.aspx
Here is an example of purchasing 4 sheets of LTO-6 labels starting with sequence 000000 with the label printed Vertically. The first label would be 000000L6 and the last would be 0000079L6 with this order.
Here is an example of 1 sheet of 20 for cleaning cartridges.