You can buy BTSX for BitShares X on several exchanges, most notably Bter and BTC38. For other DACs, owning PTS still gives you rights to future snapshots, such as the Music DAC which should be coming in the next few months. PTS is available separately on exchanges: those same two, plus Cryptsy and Poloniex. I wouldn't recommend buying BTSX on Poloniex unless they've fixed their issues.