I just bought cheap CD/DVD and book shelves. The sort that are dirt common and you can buy at Walmart or whatever. I then assembled them but putting the top of one unit on upside down. Dowel pins hold em together, so you can stack em. I made doors with plexiglass, adding small hinges to hold them on to the shelving units. Kept the dust off nicely and looks a bit cleaner overall.. Here are some photos so you can get the general idea...
All of these are TWO shelving units, stacked as I said:
Same with the white bookshelves here, two units stacked. Cheaper to buy that way than one large one (read below):
With the white ones, I learned my lesson a bit. If you assemble one shelving unit ENTIRELY upside down, you can stack em with dowels and it'll give you an extra shelf. These units come with a small "board" that fits across the bottom so there's no space at the bottom. I did not install that, hence the extra space once stacked. I learned that lesson after that photo was taken so you can't see what it looks like... It looks like one solid unit though, like it was meant to be that way.