I purchase from Playle's Collector Supply on Ebay at http://stores.ebay.com/Playles-Collector-Supplies They seem to have better prices and I have always had a good experience with them. Their feedback score is 100%
You can also buy directly from BCW and save if you buy in large quantities. Even though I use a lot of sleeves their shipping costs are killer so I typically will use Ebay to find the best deal. But you can check them out at http://www.bcwdirect.com/.
Other useful storage items are photograph boxes (found at Michael's) and Postcard Album Pages (found at BCW or Playles Collector Supplies)