how or where is best to generate an isbn code for In Design?
There are scripts that can make EAN-13 barcodes for InDesign, but I think the service offered over at Bookow.com is worth a look: https://bookow.com/resources.php#isbn-barcode-generator
Otherwise there is a script available from Adobe Add-ons that works well but requires typing the ISBN above the code afterwards:
And there is the BookBarcode script by Indiscripts that has a trial and paid version: https://www.indiscripts.com/post/2017/06/bookbarcode-2036-patch-for-indesign-cs6-cc
Hmm, do you really want to generate a barcode (perhaps for an ISBN you already have)? Or do you actually want to obtain an ISBN code? If you want to get an ISBN (or ISSN) code see https://www.bl.uk/help/get-an-isbn-or-issn-for-your-publication