You can buy either ferrous fumerate, ferrous sulphate, ferrous gluconate or ferrous bisglycinate off prescription.
Easiest place to buy it is online from Amazon - make sure the supplier is a pharmacy. Or you can buy it from a behind the pharmacy counter but it is up to the individual pharmacist whether they sell it to you once you have told them you are low in iron and have been advised to purchase some as it is cheaper than on prescription.
Ferrous fumerate and ferrous sulphate are the cheaper iron salts at <£5 per box/tub and those are what the NHS prescribes people. Ferrous gluconate and ferrous bisgycinate (Solgar gentle iron) are more expensive at >£10 per box/tub. Just make sure you don't buy Ferrograd or Floradix as they don't contain enough iron in them to raise someone's levels if they are sub-optimal and they are also expensive for what they are.
This link is worth having a read - patient.info/doctor/non-ana...
Under management it states the treatment is 1 x ferrous sulphate twice a day or ferrous fumerate, ferrous gluconate as they are better tolerated. Like @Clutter states take each tablet with vitamin C.
What is missing from the advice is:
1. Ferritin levels can take a long time to increase
2. You should take iron tablets on an empty stomach with only vitamin C and water. So take it two hours away from other food,drink and supplements to avoid interaction. If you are on any medication it should state on the medication how far away to take it.
3. You must test your ferritin level regularly e.g. once every 3 months as too much iron is poisonous. Unfortunately you are going to do this privately. Private testing information thyroiduk.org.uk/tuk/testin...
4. Once you do get your ferritin level up above 70 you need to go onto a maintenance dose to maintain it. Some people find a couple of iron tablets a week is enough to keep their level up while others find eating liver once a week does this, while others have to do a combination of both.