A film positive, sometimes referred to as a transparency, is what is used to create the screen stencil using photographic emulsions. A professional transparency is a clear film printed or thermoset with an black opaque image. Having said this there are, as always, many different ways to achieve the effect of a film positive without the cost.
Anything that holds out light will work as a film positive.
If you understand this basic premise, you can make just about anything into a film positive. Here are some methods you can use :
Laser printed transparencies allow you to create a cheap film positive from a computer file or scan. You can buy blank transparencies, suitable for laser printers, at stationery stores. The one drawback is that laser printers use toner, and the density of the image may not be good enough to effectively hold out light for a decent exposure. There is a product called Toner Density Spray that you can apply to your transparency after it has been printed and that will darken the toner and make it more reliable as a film positive.
A photocopier is another good alternative for creating cheap filmwork. Use blank transparencies to photocopy your image onto the film. Again, the use of toner density spray may be necessary. Another thing you can do to make your positive more opaque is to photocopy two transparencies and lay them over each other. Perfect alignment is difficult to achieve with this method, as photocopiers, as well as laser printers, use heat to create the image. There will always be a degree of shrinkage in your transparencies, but this method can work well enough.
Tracing paper also works a treat as a film positive. It is clear enough to let through the ultraviolet light, so if you ensure that your design is dark enough to hold out the light, then this is a good method. You can use this if you do not have a computer, or would like to retain a hand drawn feel to your design. Opaque pens are great, usually they are available from art stores in different tip thicknesses and such. You can use these to draw/trace your image onto the paper. Opaque pens contain a red ink, and under UV light, red is as powerful as black.
Anyway, this is just the tip of the iceberg. When you think about the statement above, you may want to experiment with everyday items that are opaque enough to hold out UV light. Mesh bags like those you buy oranges in can create a nice effect. Pieces of paper, masking tape, leaves, etc, can all be used to make some sort of image. As long as light cannot fuly pass through it, you will get some sort of effect.