So barley, oats, rice, corn, millet, spelt, amaranth, quinoa, rye and sorghum are right out! Maybe spelt (Triticum spelta) would be OK.
Transubstantiation only works with wheat?
Correct, wheat host only.
The essence of a sacrament is that it must signify. Jesus compared himself to a grain of wheat. Jesus instituted the Sacrament at a paschal meal, in which the unleavened bread is made of wheat flour. Barley, oats, and corn do not signify Jesus, so it’s wheat and nothing but.