You can get a trickle charger at any auto parts store, marine store or Wal-Mart.
If you insist on a genuine B&S charger, you can get one at http://www.jackssmallengines.com/ with part #B4177GS. This is a standard charger used on most B&S gensets. The cost is about $25.
Don't forget to click Accept so I can get credit for my time, and please consider leaving feedback, as this is how we are rated.