At the moment, without the Marketplace, it is only possible to purchase the packs during the weekly drops, which contain PIX in random locations, based on the regions included in each drop.
With the introduction of the marketplace to the game during launch, it will then be possible to bid for specific PIX that are owned by other players. In the same way, you'll also be able to sell your PIX to other players.
Please note that in the Marketplace, the bid needs to be accepted by the seller in order for the transaction to be concluded. The team behind the game cannot transfer the PIX or help with the buying process.