Sonifresh via Amazon offers its Sonifresh Replacement Toothbrush Heads 4-Pack for $13.99. (Select the "one-time purchase" option.) Coupon code "3YU7VJ5X" cuts it to $6.30. Plus, Prime members receive free shipping. (Non-members can order via Subscribe & Save for free shipping.) That's tied with our mention from a month ago, $20 off, and the lowest price we could find. They're compatible with a range of toothbrushes including Philips Sonicare FlexCare, EasyClean, HealthyWhite, and Diamond Clean.

Update: The price has dropped to $11.99 before coupon, $5.40 after.