Where can I buy STE or А2А tokens after ICO ends?

The ICO is only the initial stage of tokens circulation process. After the ICO ends, you can buy the tokens from the 50X Exchange directly. 

STE: at first STE tokens will be available for purchasing on STE Shop and on the exchange. The difference is that if you purchase STE in the Shop the tokens you get will be Primary, meaning they will receive income from both 20% and 80% income pool. If you purchase tokens from other people using the Exchange the tokens will be Secondary, meaning they will be participating in 80% pool only.