FineTech007 via eBay offers the Raspberry Pi 2 Model B USB 802.11n Wireless Adapter for $4.63 with free shipping. That's tied with our mention from two weeks ago and the best deal we could find by $2. It supports 802.11e, h, and i protocols and encryption.
Various merchants via eBay cut up to 85% off select refurbished tech items, with prices starting at $9.99. (Although the banner says up to 60% off, we found deeper discounts within.) Plus, all orders qualify for free shipping. Warranty information is available on individual product pages. A couple best bets, with comparisons to new units: