I am not sure where to buy the adapters for the car sear for this stroller and which car seats would fit.
You need to ask the seller you bought the stroller at, or any store that sells this model, to order the adapters for you. They are normally ordered with the 2in1 or 3in1 set, as far as I know. They are not that easy to get by just so.
There are usually 2 types of adapters, one for the dedicated, no-name matching car seat (I don't recommend that one), or Maxi-Cosi/Cybex type of attachment system, which can accommodate Maxi-Cosi, Cybex, Nuna, Joie, Recaro, Kiddy and more car seats with the same attachment system. Again, for the adaptors' availability, you need to ask the seller(s) (or directly the manufacturer).