Each brand of injection molding machine is a little different, so you need to make sure the mold you make is compatible with the machine you're going to be using it on. Something as large as a motorcycle fender is going to take a very big injection molding machine. They are classed by the amount of plastic they will deliver in a single shot, so weigh your largest intended product to see how large a machine you'll need, and find someone local who has one. They will be able to give you the specifications for the mold it wants. China does offer very inexpensive mold-making, but make sure you can communicate well with the company that's making them for you, since mistakes will be costly.
Here's a site that offers good free information on injection molding; their "whitepapers" are a good guide to some of the issues involved: https://www.protolabs.com/resources/whitepapers