Seconding iron bisglycinate.
DD was prescribed 200mg ferrous fumerate by the GP when her blood tests showed low iron/ferritin. She found she got digestive issues on that. I looked for more absorbable forms of iron and iron bisglycinate looked like the best from that point of view. (Apparently it's unabsorbed iron which causes the digestive issues which many people get with iron supplements - so the more absorbable/bioavailable it is, the better, from that point of view.)
Iron bisglycinate was approximately twice as absorbable as ferrous fumerate, according to the studies I read. So we started DD on a 50mg dose (just to check whether she was OK on it) and then upped it to 100mg, and then 150mg. Her iron and ferritin improved well on that, although it took months, and she had no bad digestive effects.
We got ours from Bodykind but I'm sure there are other good possibilities:
www.bodykind.com/supplements-c11/minerals-c52/iron-c97/vega-vitamins-iron-bisglycinate-60-x-50mg-capsules-p1930
Other stuff I found out while looking into this:
Taking it with vitamin C improves absorption.
Taking it with anything containing calcium (e.g. milk products) impedes absorption.
The latest thinking seems to be that taking it every other day, instead of every day, actually gives better results, for some reason. (Same dose, not doubling the dose). This seems really weird to me. It does appear to have scientific backing, but to be honest I just ignored this because it seemed so counter-intuitive, so I got DD to take the supplements every day. But thought it was worth mentioning.