I have published some story books for kids and I have already bought the ISBN numbers for the same. But I am confused on how to get those ISBN numbers as barcodes on covers of books. What is the cheapest way to do this? Please advise.
You require the barcode as an image that you can place on the cover just like (for example) the thumbnail picture of the author.
There are plenty of free software packages that will generate barcodes in different styles. Be careful that the one you choose "understands" the requirements of ISBN.
Online Barcode Generator provided on a web page by Terry Burton seems to do the job, but if you don't like it, a Google search will find you more options.
Try checking the output it generates for the code on a book you already have. Then you can compare the result with the barcode on the book in your hand.
This is what I understand from your questions, you already have the ISBN numbers and now only thing left is to get those numbers barcoded on your books. Check whether your publisher can help you on this, else contact a reliable barcode reseller. I googled I came across this ISBN Barcode guide - it would make your job easier.