As stated above - Judo has it well, though some Judo dojos are better than others. Anyway - just try to move a standing uke
in 8 directions - front, back, left, right + four corners. Try with uke
, then in right and left posture etc. Then start to apply it to Aikido. That is more or less how I started out in Aikido, which was a mix of Tomiki style, plus Kyushin (from the Kenshiro Abe line). Oh, plus Judo of course. These days I rarely, if ever, see balance taught in any kind of systematic way.
Don't usually like self-advertising but - I have a few ideas you could try here: http://discovering-aikido.com/balance.htm
(There are many more)