Andy, You get one sketch from tinker every day (when daily crew roster is reset)
Tinker is the old guy on extreme left of your game advisers.
Sometimes he gives duplicate sketches which you can trade in market for the needy one's.
Additionally join a BH, they have sketch bank normally and they will give you your wanted sketches as soon as they could.
Maximum limit of free sketch transfer between 2 players are 2 sketches, while if you exchange it through market, there is no limit on it.