I have severe iron deficiency anemia, I can't take ferrous sulfate po bid due to constipation. What other oral iron agent can I take that?

Alternatives. Try what dr. Olson said- profe- and see if you could tolerate. If not, intravenous iron infusion would be an option to treat your severe iron deficiency. Intravenous iron will by pass all the GI problems including constipation- so that would be a good option for you.
Profe. Iron is hard for the body to absorb, so the left over causes constipation. The rx iron profe is a protien coated iron that gets better absorption and causes much less constipation.
Add probiotics . I will not guarentee this will eliminate (no pun inteded) the problem. But a large (10 billion units) of probiotics to aid in improving the digestive flora may help the digestion and possibly aid the constipation problem. Probiotics are otc. Gnc has a good brand but most drugstores will carry them.