There are different commercially available pre-filters on the market for almost every model. However they are such a simple design, that there is no reason not to make one yourself.
Why use a pre-filter? Well, it depends on the quality of the water. If it is a clear water supply, then it is not needed at all. It should be used when your water source contains a lot of sediment and particles. The pre-filter serves to remove some of the larger particles, so they do not clog up your filter. Make no mistake; a pre-filter will allow all sorts of particles to go through. Think about a coffee filter. When you use one in your coffee maker, it removes the grains, but the coffee is not exactly clear drinking water.
Here is how I made mine. I took a small plastic container (it came from a chocolate egg in which there was a small toy). You can use a film canister to do the same thing. In one end make a hole into which to fit the filter hose. Make sure it is a tight fit. On the other end of the container make several holes. Now take the end of the filter tube, and wrap it in coffee filters or whatever material you wish to use. Close it, and you are good to go.
In the field, you can disassemble it, and clean the dirt from the filters, or can just replace them if they get very dirty. When not using the pre-filter, make sure you let it dry well so it does not become a breeding ground for bacteria.