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:
Looking for one here as well. Obviously Mitch gets first dibs... I won’t need one for at least six months but just wanted to start looking in case anyone came across one.
I did also check with AvFab. They sell one for 1700. Takes about three hours to install. The only disadvantage is that The STC is only good for a single plane, so you can never move the seat. Probably not a big deal. On the plus side it’s brand new and any color you want.