Tesco sells 150g packets of chia seeds for £1.50. Not sure what the RDA for a horse is, but I sprinkle a dessert spoon full on my natural yoghurt, blueberres and raspberries.
I feed chia but I buy from amazon, 4kg works out most economical as I need to feed around a 1/2 cup/full cup a day. It's the only thing that's really made a difference to firm up soft droppings/suspected hindgut issues for my horse, gives a nice shiny coat too.
From my understanding, it works similarly to psyllium husk forming a gel in the gut.