Home Depot has 3M Aqua-Pure AP811 Whole House Replacement Cartridge (Model #5618904) for $13.42 w/ free store pickup.
- Height: 9-3/4 in. (24.8 cm) diameter: 4-5/8 (11.7 cm)
Notes: the product title & the model number matches AP811; but the picture shows AP810 (blue)