Depends what stage of the grow you are at and if you have cal/mag def yet.
I personally don't buy any of the cal/mag feeds. I use a combination of crushed eggshell in the soil and molasses in the feed.
Eggshell should be washed dried, and then ground into powder using a pestle and mortar, and then mixed with the soil before potting up.
If the plants are already in pots you can poke holes all around the plant like numbers on a clock face and sprinkling the powder in the holes and over the top of the soil then mix up the top cm of soil and water the plant to fill the holes.
It is very important to crush the eggshell as fine as possible because the bigger the pieces are the harder it is for them to be absorbed
So the eggshell being 100% calcium take care of half the problem and to take care of the mag I use molasses in the feed
You should use either blackstrap or regular unsulphered molasses. It should be added to the feed at a rate of 1teaspoon of molasses per litre of feed, and should be used every other feed for the whole flowering cycle.
I have found that using molasses gives a better depth of flavour and also increases the yield, so I would recommend that, if you don't already use it, try it even if you get a cal/mag supplement.
If you have a cal mag def showing then definitely get a supplement
I hope this has given you another organic option to consider
