Card packs and chests are only available from the website during seasonal sales periods, and can be purchased using ETH, $GODS, or other currencies. When a sale is active, you can purchase packs here so long as you have money in your Immutable X balance (see: Adding funds to Immutable X).

When a sales period is over, you can purchase the seasonal cards you are looking for (as well as chests) from other players on any Immutable-powered marketplaces. On occasion, you can also earn seasonal packs from in-game events such as Weekend Ranked.