Most assemblies are paid by the District or by the PTA/PTO.

But here are some additional ways to help you fund the assembly of your choice.

http://donorschoose.org - 
Lets public school classrooms post information about classroom projects that need funding.

http://adoptaclassroom.org - 
By adopting a classroom, donors form partnerships with specific classrooms providing financial and moral support.

http://grantsalert.com/ - 
Provides lists of foundation and government grants for educators.