If I remember correctly, the horse's gut adapts and starts to digest the psyllium after about 10 to 14 days, at which point it no longer functions as bulk and won't clear the sand out of the gut. It can still be beneficial to feed to help support a horse with digestive issues. If you want to use it for clearing sand, it is probably most effective to feed it 10 days in a row, once a month.
It can be purchased online in bulk--I don't remember where I got it.