Almost similar to 16mm there are re-loadable super8 cartridges available. It does not work as easy as for 16mm but you can do play with it.
You can find these on eBay, but I think that they might be too expensive when you find them there. The fun part of this re loadable super8 cartridge is that you can expose short parts of film that you wanted to use for titles or to test you new 2nd hand camera you just bought from the flea market :-) You should have some patience to happily use the cartridge.
How it works.
The illustrations below show what you need to do. First you would need a unexposed part of film that you could take out of a Tri-X film. Follow the images from the left to right. In the first 3 images they show where to put the film. Remember to do this in full darkness since Tri-X is panchromatic film [sensitive to all light].
Image 4 shows the film in the cartridge. And number 5 + 6 show how to load the film in the rest of the cartridge.
Drawing 7, 8 and 9 show the other side, how you should fold the film in the cartridge.
When you are at number 10 you can close the full system to make it light tight, and by 11 you are ready to use it in the camera. Be aware to expose the film manually. The cartridge might give wrong information to your camera about the actual film speed you are using. So this all looks very easy but the trick is to fill the cartridge without the film to break or damage during the shoot of your scene. So good luck, a new challenge for super8 lovers.
If you load un-exposed film in a empty cartridge you make sure that the emulsion side is on the right side, that the winding of the exposed film is correct and that the perforations are on the good side, otherwise it will not be possible for your camera to transport the film forward.
If you realize how much is involved in loading a cartridge it is amazing that a factory at Kodak can do all of this fully automatic.