Is it possible to find a 1965-1969 (shape) V6 Mustang convertible ?
No they did not have a V-6 (stock).
One could be swapped in easily enough.
But the stock 200 Ci (in-line 6) engine was fun enough in those cars. A 289 (V-8) was a common engine also.
The preferred models were the big blocks and the Boss engined ones.
A restored one will not be cheap, and it is expensive to have done (or to do yourself). Over 80% of the ones that some tries to restore do not get completed. Because either it took more money then planned, skills needed were beyound the hopeful or interest in the project was lost.