I think it is wonderful that there are so many options for bulletin board backgrounds. I bet you could add to this list, but here are a few that i have found work pretty well:
*Tablecloths (If you have light colors you would need to double them, otherwise you'd be able to see the cork board behind it).
*Sheets
*Fabric
*Wrapping paper (I can find solid colors at my local dollar tree).
*shower curtains
*or of course you could use the paper designed for bulletin boards, if you have cheap access.
and you can make your own border or use pre-made, i usually can find some at the dollar tree for cheap.
My very first bulletin board was on weather:
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The makings of the board:
Background: Blue wrapping paper.
Clouds: Pillow batting, with tons of staples :)
lightning & rain: Using yellow card stock cut out lightning rods and for the rain using silver christmas rain-like-tinsel from the dollar tree. staple it under the clouds.
Sun: Huge yellow poster board, cut into a half circle and then long skinny triangles to make the sun rays.
Wind: White poster board cut into swirly shapes with grey marker in the middle. Stick the biggest one like its coming out of a cloud :)
The thermometer: Using white card stock paper cut out a rectangle and outline it in red, use a real thermometer as a guideline for drawing.
On the board i have a place where they can place what the weather is today... and off to the side i have different elements of weather listed :)
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REALLY THOUGH, JUST HAVE FUN WITH IT! :)