If you have some flexibility in your travel dates (a few days) then there is no problem finding available A inventory. Often I am constrained to fly on a specific flight (not just date) and that makes getting A difficult at times. D is sometimes easier and sometimes harder (varies by route) but often there are many more D-only flights. In general D is considerably easier to reserve than A (worst case I've had an AONE3 which had 2 sectors in A, the rest were in D). In general A availability is an order of magnitude better than Z (award first class). I think it is true that they will always sell you A on any flight with Z availability.