When you have two or more mating surfaces, you are going to need some type of seal that is going to take care of the junction between these two surfaces. In most applications, a gasket can fill the role. It is a piece of rubber, or a ring, that you can place in a certain spot between the two surfaces in order to seal the junction properly. The place you will normally see a gasket used is within various types of engines, because they have so many parts that are working in conjunction with each other.
Depending on the type of engine you are using, or the type of application you need the engine for, you are going to either order a gasket that is already on the market, or you will need a custom gasket. The process is a lot simpler if you have the type of engine where an aftermarket gasket can fit. All you have to do is get the exact type of gasket that was originally found in the engine. If you do not know the type, you can always look it up or ask the manufacturer. But it is not always so simple to find the right gasket.
In some situations, such as custom-built or very rare engines, you may not see a gasket on the market that can do the job you need. But it is not a reason to despair. All you have to do is talk with the gasket manufacturer, and they can make something completely customized for your situation. They will need some information about the engine you are going to use, along with detailed pictures of where the gasket is going to go. In some cases, they may even want to see the engine in person. But they should have no issues building your customized gasket.