Tomas's Journal is a note carried by Tomas who can be found between Nipton and the Nipton Road Pit Stop. The note can be obtained either by pickpocketing him or by killing him.
Transcript
Things finally seem to be turning around. I made a nice bundle of caps turning in some scrap in the last town I passed, and now I have a new traveling companion. Her name is Jacklyn, and she's pretty cute. I'm not going to get ahead of myself here, but things are looking up! And to think, I was actually beginning to doubt the power of my lucky necklace!