Sonifresh via Amazon offers its Sonifresh Replacement Toothbrush Heads 4-Pack for $10.99. Coupon code "LOL46LOL" cuts it to $5.93. With free shipping for Prime members, that's $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.