Most of the larger on-line shops sell Addis.
One very good one is Yarn Forward in Toronto.
Their Web site is at www.yarnfwd.com
There is also a gentleman in the UK who sells Addi needles at large discounts. Go to ebay.com and enter "Addi needles" in the Search box.
The seller's name is Jeff Wonderland and he usually has a good selection of sizes.