SP comes directly from rewards or buying STEEM and Power Up.

Sometimes people get STEEM from exchanges.
Sometimes they use BlockTrades.
Sometimes they use the internal market and sell SBD for STEEM.
Sometimes they buy it directly from someone with another currency.
Sometimes they already have STEEM and decide to Power Up.

When more users power up, then it reduces the supply of STEEM. If enough people do it, then the supply can become small and demand will go up for the limited supply.