http://www.youtube.com/watch?v=aXZo3phy56E
As you can see at this event, we used poster cardboard from the dollar store, and I used a 2 1/2 stencil set I bought plus a black marker.
Altogether it was maybe $10.00 (because of the initial cost of the stencils) and I have been using the same stencil set since 2006 (I think they are cheaper now). Have to buy a new marker after a few signs.
I now use 2 signs, one on each side of a poster matte board. I just make new posters and change on the matte board. I am sure in the Los Angeles area, you can probably find really big pieces of cardboard and tape more than one poster paper side by side to make a banner, then just fold it instead of rolling it.