Card packs and chests are only available from the website during seasonal sales periods, and can be purchased using ETH (to learn more about purchasing and ETH see our Purchases and wallets section). When a sale is active, you can purchase packs here so long as you have money in your Immutable X account (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 the Immutable X Marketplace or approved third-party marketplaces. On occasion, you can also earn seasonal packs from in-game events such as Weekend Ranked.