From the Gnucash 2.6 Manual on "Find":
Exactly which transactions are searched depends on where you invoke the tool from. If you start from the main accounts hierarchy page, all transactions will be searched. If you start from an individual account register, only transactions in that account will be searched. And if you filter the transactions in a register using View → Filter By... , then only transactions in that account and shown by the filter will be searched.
All you need to do is start your search in the main account hierarchy tab, and the Find feature will search through all your transactions.