Actually any well balanced granular fertilizer will work. I use a generic 13-13-13 in the band in my EB's with great results. I do find that a late season additional feeding also helps but I just use a liquid for that.
If you don't like the idea of synthetic fertilizers you can make your own with 1 part each of blood meal, bone meal, and greensand. It works ok but isn't as long lasting as the synthetics so will need to be replaced mid-season.
Enjoy your Earthboxes. Have you read all the online info on how to make your own? Less expensive and hold more water too.
If God didn't mean us to garden <i>she </i>wouldn't have created dirt.