In my place they make koliva using rice as it is difficult, in fact, nearly impossible to find wheat grains.
Where in the world are you? Wheat seems to be an almost universal grain. Rice just does not have the same theological beauty as wheat.
Many parts of Russia, for example, cannot grow wheat, as it is too cold. Even rye struggles in some of the more rugged areas. Many Russians use rice instead of wheat for their kollyva, particularly those from the regions close to China, and those who did indeed come from China, such as the Harbintsy.
Wheat is the ideal, historically and otherwise, but sometimes one must make do with what one has. Better to make a sincere offering, even if the ingredients are not conventional, than make no offering at all.