Iron (Fe) is a very important mineral required by cats and dogs. It has various roles including being involved in the formation of hemoglobin (together with copper) that helps in oxygen transport as already noted while looking at functions and cat food high in iron.
Too much of this mineral will result in feline iron toxicity and this typically happens if products containing it such as multivitamins and pills for human or it is over-supplemented.
On the other hand, too little including in diets (on rare occasion) or other cause such as underlying conditions including excessive blood loss can lead to deficiency symptoms including iron-deficiency anemia. In such a case, your vet may recommend supplementation.
Under normal circumstances, supplementation is not necessary since there are many cat foods with this essential trace mineral including organ meats, fish, shellfish, etc. Also, most of the commercial brands have it added to ensure they have the required amounts.
However, due to gastrointestinal disorders and other factors, Fe absorption may not be efficient. Also, other factors may affect this animal’s ability to retain this mineral. Such cases may require extra amounts to be given.
Supplementation is done both by oral means where supplements with bioavailable forms of Fe are used or parenterally including via intramuscular (such as using iron dextran), intravenous and subcutaneous injection.
Since excessive amounts can lead to toxicity as already noted, talk to your veterinarian before you begin supplementation.
The best iron for cats supplements
There are many brands in the market include those that come in liquid form, as capsules or tablets or solution forms for injection. Your vet may recommend some of the below brands to help deal with deficiency or iron-deficiency anemia.
1. Pfizer Animal Pet-Tinic Vitamin-Mineral for Dogs and Cat
It has iron, copper and five essential vitamins (thiamine, riboflavin, niacin, vitamin B6 and vitamin B12) which will be easy to administer since it has meat flavor making it quite palatable.
2. Liqui-Tinic 4X Concentrate
This is another good source for iron for cats with anemia or deficiency symptoms that comes in liquid form which has vitamin B complex and amino acids.
It features a palatable formulation and will support, puppies, kittens, cats, dogs, cattle, horses, swine among other animals and boost Fe absorption.
Dosages are 0.5ml per cat weighing up to 3 pounds, 1ml if it weighs over 3 pounds or 1 teaspoon if your feline friend weighs over 20 pounds given twice daily. 
3. Tomlyn Nutri-Cal High-Calorie Dietary Cat Supplement
It is not an exclusively iron supplement for cats but has this mineral and others and will ensure your feline friend’s optimal health.
Normal dosages are one and half a teaspoon per 10 pounds of body weight daily if your pet is not eating and 3 teaspoons if it eats.
4. Zoetis Pet-Tinic®
This is a daily liquid iron supplement for cats and dogs have a meat flavor making it quite palatable. It also has a copper and other five other essential B vitamins (thiamine, riboflavin, niacin, vitamin B6 and vitamin B12).
Dosages for Pet-Tinic is one dropperful (1ml) per a cat weighing 10 pounds given twice a day and for kittens, the amount should be half a dropperful or ½ ml per 5 pounds of body weight twice a day.
Where to buy them?
If you are interested in cat iron supplements, try them in your local pet stores or online stores including amazon.com and amazon.co.uk, eBay, Chewy, PetSmart, Walmart, among other places.