Best source is Mitsuwa, available fresh, actually it's all from 1kg frozen packages from Alaskan salmon whether you purchase from Mitsuwa, Sea Ranch or Browne Trading. There is no grading that I know of except in the mind of the purveryor.
I purchase in frozen 1 kg packages from Mitsuwa when on sale and repackage in those small plastic throwaway containers sold in grocery stores. Keeps well for 6 or more months.
It's currently on sale at Mitsuwa and is usually around $30/# at this time.
Do not purchase any in those little glass jars sitting on your grocers sheves, they are pasturized and heavily salted.
I have never seen grades of Ikura or salmon row but there is a Japanese variety that is prepared with shoyu. It is delightful, expensive and if used for sushi preperations, needs no additional shoyu.
It is sold every year at the Hokkaido Fair held at Mitsuwa and its over $60/#.
Once in awhile it is availble from another Japanese source at Mitsuwa in the $30/# range but unless an Ikura devotee, stick to the normal product.-Dick