If nobody has one for sale, you might can try the salvage yards or ebay. I got mine from Dawson Aircaft (aircraftpartsandsalvage.com). There is company that sells brand new ones also, but I believe if you go that route, the seat is stuck with your airplane. If you buy an actual Cessna child seat, it can be put into any 150.

N8395M 1970 A150K
The Carolina Swampfoxes founding member
I've landed my Aerobat in the following states:
[Linked Image from visitedstatesmap.com]