Okay, so you couldn't find any Jerulsalem Artichokes to grow (which also have inulin). What about Dandelion roots? If you go out into your back yard you'll find plenty, not only absolutely free, but readily available to anybody with weeds!

No planting, just look for the hallmark yellow flower and dig it up!

If you really want to gourge yourselves on inulin, ask you nearest friendly farmer if you can weed his paddock for free!

Should get a couple of wheelbarrows full, no sweat! (Well, maybe a little bit of sweat on a hot day).

Happy weeding!