First, you'll need arts, and education grants, and local, as well as other forms of sponsorships.
Second, you'll want to determin wheather to make theremin foundation a nonproffit organization which benifits others, and or provides for the up-keep of historically important instruments, and or theremin related training scholarships, or all of the above, as SPEBSQSA does, or make it a revenue earning organization.
How wide spread dow you want the foundation? Do you want chaptors to form throught the U.S., and or internationally?
A set of gaols, and bylaws which govern the organization will eventually be called for.
In SPEBSQSA, there are local level chaptor performances, and functions, as well as national level competitions, conventions, and anual training classes for officer level, and chorus level participation, known as COTS (chorus officer training school). These training programs are designed to train directors, as well as support related levels of office relating to the operation of local, and national level organizations. I've been to quite a few in my time.
Both SPEBSQSA, and The New York theremin society sounds like great groups to consult about further advice on forming a theremin foundation.
I'd certainly be willing to pay dues if I knew they would go to keep a theremin museum going somewhere, or help train a serious thereminist, or bring someone up to speed as an instructor, or help someone keep an vintage theremin going.
This is not something to take lightly, nor is it going to be an easy thing to start, let alone keep going, but I think it's time has finally come.
I wish you, or whomever attempts to bring this dream into the realm of reality all the best luck with such a venture.