At least in the US, the names Grade B and Grade C have already been phased out. Some states had done so earlier, but the USDA officially renamed them earlier this year to wordy variants of Grade A. So if you are looking for Grade B, you now want "GRADE A: DARK COLOR & ROBUST", and if you were so ahead of the game that you wanted Grade C, you now want the eloquently named "GRADE A: VERY DARK & STRONG".
This is an accurate description of the legal US grading system as of March 2, 2015 (under 7 U.S.C. 1621-1627). For anyone interested, here's a link to the official USDA guidelines (grading is §52.5962 on page no. 2, which is actually page 4 of the PDF):
[1] http://www.epicurious.com/ingredients/grade-b-maple-syrup-bu...