Well, I was wondering why someone resurrected such an old thread, but here:
http://www.ebay.com/itm/Crystal-Cover-Hard-Shell-Case-Clear-For-Nintendo-3DS-XL-LL-/370896710737?pt=US_Video_Game_Cases_Bags&hash=item565b2aac51
I use cheap clear plastic cases. They typically last quite a while...