Decals bought online or in a specialty car store deliver the best bang for your buck while still looking professional. There are several websites that specialize in decals. The prices for full side-door decals (such as the one in the picture) can range from $50 to $200. Do a simple Google search to find sites with hundreds of options.
Getting the car painted is another good option. Car shops can sometimes create fancy designs like racing stripes or flames to mimic the ones from "The Fast and the Furious." This is a more permanent option and may cost more money than buying decals, which you can remove.
Shop at Michael's arts and craft store or elsewhere to find sticker kits. Car shops may have do-it-yourself decal kits if you do not feel like spending as much money on a premade decal online.
The cheapest option, and the riskiest, is to find a large stencil and paint your car yourself. Some websites can walk you through different designs or tutorials, but the risk is that you will not be able to make it look entirely professional. Certain areas will end up having thicker paint and other areas may appear streaky, especially if you use spray paint.