Use PowerPoint to create giant letters in any font. Use pictures, textures, or color to fill to fit any theme. You can also print out on bright paper (and laminate) for cheap and durable bulletin board letters. Great way to have just the letters you need.
I've done this bulletin board every year, and the kids (and adults) love it! I send out a letter to the parents asking for random items, and this is what I got this year:) The item to search for changes every day.