I was not able to exchange ETH for BNB on trust wallet. Not sure on binance. Others have said they can do LTC to BNB but I have not been able to do that.

That really is the key. If you have other coins, you need a way to exchange them for BNB which you can then send to TW and exchange for SM. I was alerted to Atomic Wallet, and it seems super easy to do exchanges there, but they have halted all exchanges for BNB currently.

I have to imagine all these exchanges are getting hammered and overrun with asks for BNB