What kind of custom car decal? You talking original decals from car and parts makers? Should find them at those car accessory shops that dose custom work on your car. But if you are talking about specially done ones just to fit your car body..etc then are you providing the design and artwork? If you do, there are some shops at Bras Basah Complex that can take your artwork to make the decal for you.
If there are a few colours then you might need to make multi-layer and paste them together to form your decal design. If it is a picture then you might need to get it printed on special weather proof stickers and have it cut to shape.
Unless you are familiar with designing with design program like illustrator, photoshop..etc you might need a friend to prapare the artwork for you. There is also the issue of copyright too but I am not sure how far some of the brand owners will enforce it. But if you are making your own flame design, text or personal graphic design for example to paste on your car then I think that is fine.