I am afraid that ultracentrifugation is quite unique for the efficient separation of nanoparticules of different size dispersed in a solvent. It is hard to replace it for that specific purpose.
If your particles are magnetic , then some magnetic field gradient might be useful for separation.
Note also that such ultracentrifuge machine may exist in labs (nearby your place) that are not dealing with nanotech such as biology/biochemistry labs. It worths checking whether you could have access to some equipment there.