Hi BUffy!
I am using a childseat that's also a booster for Julian now, since he is almost 15 kg. It's the mothercare brand, so was quite inexpensive, think around $200. I don't think our kids can use the booster seat on its own, in case they wriggle out of the adult seatbelt, but if you get this combination seat, you can remove the top half and use just the booster seat portion on its own later...