The accepted standard supplement is 600 mg calcium and 400U vitamin 3 times per day with meals, tho I take 4 doses each day. You can get these over the counter in the vitamin dept anywhere in a combination tablet. I don't believe Tums or Rolaids is going to be as beneficial.
Even if you improve your diet, it probably won't be good enough, and you should still add a supplement if your doctor has recommended it.
Osteoporosis is no picnic - it's silent and painless - until it rears it's ugly head. I have severe osteoporosis and have had several bone fracture in the past few years. My orthopaedic surgeon said if my bones get any more porous, that he won't be able to see them on an x-ray! Unfortunately I can't take any of the prescription bone building drugs, and they're still working to find something that I can take that will work.