Are there any over-the-counter iron supplements or do they all have to be prescribed?

Freely available. The key is to find out why you're iron deficient. If you are replacing heavy monthly losses or blood donated, this is a reasonable choice. I trust that by "low iron" you mean an actual diagnosis of clinical iron deficiency rather than simply anemia. Remember that iron deficiency may also warn of stomach ulcer or tumor, colon tumor, or other illness.