Amazon offers the Braun Clean & Renew Cleaning Solution Cartridge Refill 4-Pack, model no. CCR4, for $13.99. Add it to your cart via Subscribe & Save and the price drops to $13.29 at final checkout. With free shipping, that's about $6 less than you'd pay in local stores. It's compatible for use with all Braun Cleaning Centres.
Note: Be sure to cancel Subscribe & Save once your order ships or you'll be billed for subsequent shipments.