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.
The cheapest way to generate a barcode image for an ISBN that you have been allocated is entirely for free using a trusted online barcode generator. (Disclaimer: I wrote this tool which has generated millions of barcodes over more than ten years.)
Care must be taken when applying a barcode to your design as failure to produce a barcode that can be scanned reliably at point of sale could result in refusal to stock your product or even your product being removed from sale.
Here is a digest of some of the information from this article "How do I get a barcode for my product?" written by my wife which I recommend that anybody reads before finalising their cover design.
An ISBN is represented by a special type of EAN-13 barcode symbol which includes the ISBN number printed in human readable form along the top. The ISBN number (without the dashes) is encoded directly within the barcode symbol. There is a recommended five-digit add-on code which encodes the retail price of your book which can be placed next to your barcode. Some retailers require this so you should check this with them.
ISBN numbers used to be ten digits long until 2007 when they changed to thirteen digits. If you were assigned ten-digit numbers it is always possible to convert them to the current thirteen-digit format. My online barcode generator takes care of this automatically.
Take care to ensure the correct barcode height and width on your finished work.
Every barcode must be printed with a light margin (called a quite zone) to the left and right of the symbol so the scanner can differentiate between the barcode and the graphics around it.
When using the online barcode generator choose the EPS vector format as this does not lose quality when rescaled to an appropriate size and DPI for print. If you must work with bitmap formats such as PNG or JPEG then you must generate an initially very large barcode image (set the scale parameter to 5 or more) and shrink it as inflating a smaller image will make the barcode of obviously poor quality when printed.
Some print processes drift away from ideal over time and require periodic recalibration of the equipment to maintain the accuracy of the output. Ensure that quality controls and responsibilities are agreed upon to ensure that every instance produced will satisfy the standard required by the industry.
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.