Definitely red meat. There are other sources, as PPs have named, but the body absorbs the iron from these less well than from meat (it's about the type of iron, heme vs. non-heme). They can't hurt, of course, but I wouldn't expect to see a huge difference from those sources.
My iron was also low, as were my platelets, so I made sure to have red meat a few times a week. I was a vegetarian beforehand, so it was weird, but worth it. I did notice an improvement in my energy level.
But heck, if I had that much maternity leave, I might contemplate taking it early, too, at least once you hit 32 weeks or so.
Good luck and hang in there. It's tough to feel that exhausted.