You can make an ad on kijiji or craigslist asking for extra clippings of plants. I got my tank stocked on free clippings from someone on reddit, and there's usually people on my kijiji looking to sell their extra plants. Some grow crazy out of hand and people just want to get rid of them. So try that avenue.
Another thing, go to large chain pet stores (I went to one called PetsUnlimited under the PJ's Pets logo) and asked for a "bit" of salvinia for free bc they were struck with such a pile of it which wouldn't stop growing. I got that a little over a year ago and it's low light, been growing it very well.