What other brand car seats will fit a Costco car seat base?

None. You must use only the seat that was designed for the car seat base. The seat and base have been crash tested together by the manufacturer. If you use a seat with a base that has not been approved or tested, your child could be at great risk of injury or death in the event of a crash. Always follow the seat's manufacturers' instructions and seek the help of a certified child passenger safety technician in your area by going to http://www.seatcheck.org or http://www.safekids.org/certification