Here are some suggestions or products I've used before:

Chicory root
Nearly 65% fiber, so one of the best prebiotics around.
Some drink it like a tea or coffee, others just in a smoothie or combined with other foods.
Here are two good product ideas:  Organic Chicory Root Raw C/S or Chicory root inulin powder

Jerusalem artichoke
Another top prebiotic, and to some more palatable than Chicory root.
I may try growing this in the garden this year, but I'm months away from the opportunity.
Here is a popular prebiotic mixture I've seen folks recommend:  Hyperbiotics Organic Prebiotic Powder

Raw dandelion greens
Can pick these in season, but be very careful about pesticides and any turf fertilizer, pest repellent,
or anything that has been applied to your lawn or nearby grassy areas. 
Here is a top supplement powder if you're considering something in the kitchen:  Amazing Grass, Raw Reserve Green Superfood Organic Powder with 25 Billion Probiotics, Wheat Grass and Greens