Poultry manure works well. Corn can handle rather fresh manures.
Also, fish emulsion or Milorganite are both high nitrogen.

Tasseling corn isn't going to have time to absorb slow-release organic fertilizers. You might consider some lawn fertilizer - the straight stuff, no herbicides or other additives.

I was going to comment earlier, and then thought, no, I'll just sound like I'm being rude, or mean.

But honestly, when the corn is shooting up tassels, most of the benefit from fertilizing it is lost. You really should be side-dressing or otherwise fertilizing consistently from day 1. I like to throw a high N fertilizer down the rows every 10-14 days, starting about the time the plants are six inches tall. Cheap lawn fertilizer, summer formula NOT weed and feed is fine.

I got me a bag of Ringer's - the old NPK formula - for the garden. Not cheap, though.