Bill Chisholm has obtained sufficient commitments to purchase the Celtics and his ownership group has been finalized, according to a letter sent to existing shareholders Thursday that was obtained by the Globe. In March, a group led by Chisholm agreed to purchase the Celtics at a valuation of $6.1 billion. At the time, Chisholm’s offer was not fully financed, although that is common in a massive deal such as this. But his bid is now fully subscribed. The deal will not become official until it is approved by the NBA’s Board of Governors, likely in June or July, but that is expected to be a formality.