I could send you some, but I'm not sure how it would travel.
I think it might be a good excuse to visit some local ponds. As far as I know, the gathering of small amounts of wild seed is o.k. (I'm sure someone will correct me if I'm wrong). You may have to wait a few weeks to find seed - and this will depend on the species. Water starwort may be a suitable candidate. Of course, non-flowering plants won't have seed - if you can access oxygenators, nipping off just a tiny amount (not the roots) should be o.k. This should grow and re-root in you pond. The law is a bit strict if this is not o.k., in my opinion.
If you are like me and impatient, pop to your local large supermarket and buy a bag of watercress. Stems of this plant root and grow very quickly. You could anchor suitable ones with a stone - avoid adding compost to the water. I think they'd be o.k. without any.
Anyway - I think you could have great fun experimenting
