You have two options - replace the unit inside the lamp itself (requires making wiring connections) by buying from Home Depot or Ace Hardware:

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100376048
http://www.acehardware.com/product/index.jsp?productId=1286174&cp=2568454.2632232.2632330.1259267&parentPage=family

Or, if your lamp still lights up (but doesn't dim), you can buy one that plugs into the wall from Amazon or Lowes. Two brands (Westek, and Touch & Glow):

http://www.amazon.com/exec/obidos/ASIN/B000HJDAOW/?tag=foothighblog-20
http://www.lowes.com/lowes/lkn?action=productDetail&productId=25991-62746-X-786&lpage=none