Lots of supplements offer to aid in preventing osteoporosis. However, according to Wired Science:

The researchers found that women who get their calcium from food have stronger bones than women who take calcium pills, even though the latter group had higher average calcium levels.

In an article in BaltimoreSun.com:

Calcium from dietary sources is generally better absorbed than that from supplements, which could help explain the difference, said the study's lead author, Dr. Reina Armamento-Villareal.

Those getting calcium from foods also had more estrogen in their bodies; the hormone is needed for bone mineral density.

What are some calcium rich foods? Vincent Iannelli, M.D. writing for About.com, lists these:

• Yogurt, plain
• Yogurt, fruit
• Milk, low fat or nonfat
• Milk, whole
• Cheese, including American cheese, Ricotta cheese, Cheddar cheese, Mozzarella cheese, etc.
• Milk shakes
• Eggnog

Some non-dairy rich foods, according to Algaecal.com. include sardines, salmon, broccoli and soybeans.