I get ours at Wal-Mart. At our stores it is in the section with the Pepto Bismol and stomach medicines. I have also seen it at Walgreens in the vitamin section. It was cheaper at Wal-Mart. If stores in your area don't carry it, iherb.com is a great resource for this type of thing. I get my son's L. Reuteri from them and it comes packed in ice and even with overnight shipping it is still cheaper than what I can get it for at our health food store.
Mom of Son 16 UC dx 07/02, Sulfasalazine, Imuran, folic acid, L. Reuteri, Zyrtec, Iron, Vit/Min
Daughter 9 UC dx 08/07 Colazal, 6MP, Iron, Vit/Min., Culturelle