Changing bags make popular substitute bellows. I've also seen people use cardboard boxes, where one box is just slightly smaller than the other, so you can slide them in and out. Then just wrap the boxes in a dark cloth and that should work. If the boxes are a tight fit, they'll keep 98% of the light out. The dark cloth would protect against the other 2%.
The reason bellows are so expensive is because they're thin, flexible, hold their shape, and keep all of the light out. If you're willing to compromise on some of those attributes, you can easily do it for a lot less.
I saw a guy make a large format SLR out of cardboard once. Didn't even need a film holder. He just taped the film to the back of the carboard box, suspended a mirror at 45 degrees hinged just above the film and installed a "ground glass" made of tracing paper at 90 degrees just above the film. With the mirror down, it projected the image from the lens onto the tracing paper. With the mirror up, it blocked out the tracing paper and exposed the film. It couldn't have cost him more than $30. Obviously it would be a pain to reload film, didn't have any room for movements, and wasn't very sturdy. But I kept a mental note of it for a weekend project one day when I want to try my hand at some ultra large format.