The site Eric's referring to has some great things. When I look at these, however, I can't imagine why you don't try to build these yourself. With some balsa wood and celluclay this can't be all that difficult. Look at some of these examples to get inspired, then draw the ground surface of your bunker complex on a large sheet of paper (e.g. the back side of an old poster). Next, you start cutting pieces of balsa the size of your bunker, and build them layer by layer. Make the stairs, etc... When finished, coat the balsa with a thin layer of celluclay, plaster or another product to avoid that your bunker looks like wood when you painted it. Then you add details like pieces of rusted steel and beams, broken pieces of wall (celluclay), lightning deflector wire, etc... Finally primer and paint. Weather. Count the money you've saved and buy a superb gun, a set of soldiers, and other stuff with that !
Of course, if you want to take things easier, the collection Eric suggests looks splendid !
To make your mouth water, here's a few examples !