The ability to create a model tank that looks so much like the original that placed in the right setting they would be indistinguishable is the dream of every model builder. There are many different tools that are made to achieve that end result. One such tool is the decals that come with the models themselves. You can use these decals to help create an accurate representation of the original vehicle by keeping their proper markings intact.
While you can use the decals that come with the models you also have the choice of using other decals if you feel that it would help you to better achieve your goals. One of the various sources for these decals would be a professional sign or decal maker. These people make their living by reproducing signage for individuals. You will just have to give them an accurate and clear picture of the decals you will need produced and let them know exactly what size they will need to be. They will do the rest.
One drawback to this method is the cost. It can get quite expensive to have custom made decals produced by a shop. You could also be waiting for a bit since many of these businesses have a backlog of business. If you would like to try something totally new and interesting then you might consider producing your own decals. Decal film will be a must for this project. While it can be costly it can be used to create several decals.
Make sure that you get the right paper. It must be inkjet ready so that it will not smear when you are printing it. Using a computer based paint program create the decals as you want them to appear. Once they print out you should not move them until they have had plenty of time to dry. This will cut down on the chances of smudging that would cause you to have to start over. After they have had ample time to dry you can cut your decals out and apply them carefully to your model.
Creating your own decals can be a great way to show off your talent. It can allow you to add simulated battle damage to the tank as well since you can create false "damage" decals this way as well. There is no limit to what you can do by producing custom decals for dressing up your model tank.