There are different sizes of license plate bolts as the license plates themselves have different sizes. The license plate of a motorcycle is not going to have the same size and shape as the one for a car, which means that the bolts keeping the plate in place are also going to have a different size.
Because of this, you should check what size you would need before buying anything. Truthfully, there aren’t that many differences and there is one size that is considered to be the standard one, but you may have a vehicle that doesn’t have a standard size, which is why it’s important to check it beforehand.
After you’ve done the sizing, you will certainly find the bolts that you need in any place where bolts for license plates are being sold. If you are lucky, you will have the standard size, which is the one that is, naturally, the easiest to find.