Magnesium - alternatives to Epsom salts

I'm thinking my tomatoes might be showing signs of magnesium deficiency. I'm growing in my new polytunnel, so new ground, which is pretty good but unimproved. I'm thinking the older leaves are starting to look a little yellow, although the plants aren't very big yet. Epsom salts are the thing that most often gets mentioned, but is there an alternative, like a dynamic accumulator I can grow myself, to feed the plants?

Nancy,

This what I feed tomatoes. Pelletized chicken manure, 4-3-2 NPK plus about 8.5% calcium and there may be some trace elements that are fed to the chickens as a supplement. I also put on gypsum (calcium sulfate). We have a good magnesium level in our soil so that fertility element is something I do not think about. Our soil gets very hard with frequent waterings so when I plant the transplants I dig a fairly generous hole and mix the dirt with about 1/2 organic potting soil. This allows the roots to expand more freely and the fertilizer (every 2 weeks) will get to the roots quickly with each watering. We have high calcium levels in our soil but the additional available calcium I feel is essential.  In good years there may be 50 to 75 nice WI 55 tomatoes per plant. May put on a bit of Soil Biological a couple times per summer just to make me feel good and maybe the tomatoes too.