Hi there. Although I don't keep pigs now I grew up on a pig farm, with the sows living outdoors. We grew Fodder Beet for them, I think about 5 acres and that seemed to be the right crop. We also had a Mill'n'mix to make mash, from grains, grass powder, milk powder, fish meal and so on, the proportions varying for the age of pig, with the milk powder added for the weaners (which were not outdoors).
Try contacting the big seed merchants to find who supplies fodder beet seed - they may be willing to supply small amounts, or they may be able to put you in touch with someone else who does. Or google !