They should sell the books in Malawi, have you checked the local book stores or probably check book stores that cater to the wealthy or students?
Note that the books offered may not be the current edition, but that doesn't necessarily matter. I have the OG11 and I prefer it to OG 12 because of the way the questions are arranged, in terms of difficulty level, and they repeat most of the questions. When I went to Nigeria some years back, several book stores had the GMAT book for sale because people there also sit for the GMAT exam, so hopefully the same would apply to Malawi.
In case you still unable to purchase the book, go to your test center, someone there could give you an answer to your problem.
And in regards to your main question, GMAC has not produced any GMAT book in a digital format, so you won't be able to find a copy to download online.
But while you search for questions similar to the GMAT itself, keep reseting your GMATprep software many of the questions would change which should give you extra practice (those are the best representatives of what to expect on the exam day). Also, the software allows you to review additional problems similar to what you'll find in the GMAT test itself, so that should give you an inkling as to what to expect when you sit for the exam.

Also check out the PowerPrep software, this is the older version of the GMATprep, also created by GMAC. There are also a number of free practice test to help you get used to the test format (go to https://www.beatthegmat.com/a/resources#linkcat-61 )
Good luck!