The first thing you should do when you are on a tight budget and are looking for bookshelves is to hit garage sales, flea markets and thrift stores. As long as the bookcases are the height and width you need, and are solid, buy them. You can easily antique, spray paint, or even wallpaper a simple bookshelf!
Used bookcases are easy to find, but go really fast. So check out ads early, and be among the first at the sales. It doesn't hurt to check Craig's list and eBay, either.
The old college dorm trick for making shelves is always a good idea...as long as the kids are a safe age and height and won't be climbing them. All you need is a few concrete blocks, glass bricks, standard red house bricks, or even those heavy plastic crates and some solid unwarped, inch thick planks. But if you use the plastic crates, fill them with your heaviest books to make them solid for anchoring the wood.
Ikea has a great site for inexpensive bookshelving. However, be aware that it will need either assembly or wood finishing or both. The home improvement centers and discount centers also carry shelving that requires home assembly.
If your rooms are too small for large bookcases, and you need more storage, you can make solid nightstands from an old wooden trunk. Just remove the lid, stand it on end, and add two or three shelves, and it will serve well for both functions. If your room is wallpapered, use the leftovers on the nightstand. It will make the room appear larger as it blends into the wall.