The only way to get pre-sale packs of Chaos Legion is to have a voucher (and the funds)

There is 1 voucher per pack for each of the 1 million pre-sale packs. Divided up into 30 days, it's 33,333.333 vouchers per day, air-dropped to everyone proportional to their percent of the total staked SPS that day.

Besides the early access, pre-sale buying (and thus vouchers) are the only way to get in on the exclusive Doctor Blight Legendary airdrop, to get a title, and also the only way to potentially get a custom summoner (only for the top 6 purchasers during pre-sale)