Spinach, dark chocolate, grass-fed beef are some great sources that I eat and sit well with my gut.
Also, lactoferrin increases iron absorption so you might want to look into that. It is found naturally in cultured or raw dairy like yogurt, milk, kefir, etc.
You could also supplement with it, as there are some good formulations available online.
Article on lactoferrin and its role in iron utilization and absorption
hereBest.