It is quite common for exclusively breastfed babies to be considered anemic. The fact is that iron available in breast milk is more readily absorbed than in formula...and ADDING iron artificially (like with vitamins) can actually inhibit absorption. So, it is great that you are avoiding that.
The most easily absorbed iron is called heme iron. This type of iron is pretty much exclusive to animal products. Liver, of course, but also all "dark meat chicken and pork, red meat, and seafoods (tuna, clams, oysters, shrimp).
You might be surprised how easy it is to get a young child to eat chicken liver. I would saute them in a little butter and then put them in a food processor. My daughter would eat it spread on whole grain toast or crackers. Also, I would cook lentils in beef stock and then chop the livers small before a quick saute...mix and enjoy.
Other non-heme sources are things like dried apricots, oatmeal, lentils, many types of beans, dark greens, firm tofu. These are good to have in the diet in combination with the heme sources to maximize absorption.
One thing I did was make a point of paying attention to food combinations. Calcium can inhibit the absorption. So I tried to keep dairy products for snack times and serve water or juice (vitamin C helps absorption) at meal times to ensure my daughter was getting the maximum benefit. Egg whites can also inhibit absorption.
I added a tablespoon of blackstrap molasses to her one serving of juice that she got each day.
I tried the drops, but got much better and faster results when I made a point of correcting the potential issue through her diet.
Grape-nut also has a good deal of iron. You can make a nicely flavored warm cereal with apple juice. Just let it sit long enough to soften.