grogerson: No trick. You just have to wait for the random monster generator of the game to create one where you can get it. Keep killing the land monsters and buying and/or stealing food until one appears. Sometimes this takes a while and there is a maximum number of monsters any one time zone will allow.
No ships will be generated in the "first" time zone where Minax dwells until some of the monsters already there are killed (no small task since the first ones are powerful), and in the "aftermath" time zone it's possible that ships will spawn where they can't get to you (and if the maximum spawns are on the other continents you're out of luck - it happened to me once...).
I have a little question as well. After upgrading my stats for the first time at San Antonio (using Time gates starting from 1423 A.D. on North America), I was wondering if I could still find a frigate at 1990 A.D. or if there was a way back at 1423 A.D. using the time gates. I don't want to become stuck.