This is the smallest and cheapest delivered unit I can find:
Kem-Tek 342 Diatomaceous Earth Filter Aid 12 lbs @ Amazon => $19

Shipping is free, if you have Amazon Prime OR if you order $25 worth of stuff ($6 more). I have found 10# boxes for less at Home Depot locally, but sometimes they've had it, and sometimes they didn't. If you DO get it, it lasts indefinitely, and will help remove fine particles from the water IF it doesn't go through the filter. (It will also make your filter stop up faster!).