owanikin on mainnet you can go to https://curve.fi/#/ethereum/pools/alusd/deposit which is a liquidity pool incentivized through various methods. Liquidity providers supply DAI, USDT, USDC or alUSD to the pool. You can then buy any of the assets from the liquidity pool. They get paid an APR in order to provide the liquidity.
A more theoretical answer is: You find someone to buy your alUSD from you for some market price (the liquidity pool above makes this much easier, thus making it much easier to use Alchemix)