To journey beyond Tatopani you must be able to present an Annapurna Conservation Area Project (ACAP) permit and a Trekkers' Information Management System (TIMS) card at the respective ACAP and TIMS checkpoints. If you don't have an ACAP permit when you reach the checkpoint at Ghasa you will have to pay double (Rs 4000) for the ACAP permit.
At the time of writing you could purchase a TIMS card at the TIMS checkpoints at Tatopani and Jomsom for the same cost as getting one in Pokhara or Kathmandu. As with all regulations, these rules are subject to change and it is worth making enquiries at the ACAP office and the tourist office, both in Damside in Pokhara. Pokhara is a good place to purchase the ACAP permit and TIMS card before you leave for Jomsom.