I'm 60th level, beat the main quest, Archmage of WInterhold, Harbinger of the Companions, and have yet to run into an Amulet of Akatosh. I want to complete my personal Temple of the Nine Divines in Windstad Manor.

If You are on a PC, console add the Amulet (c8911). On a PS3 or 360, it is a Random Drop and may prove difficult to find. Dwemer Ruins tend to be a good source of Jewelry. Radiant Raiment in Solitude may be worth a try, and the store does change inventory.

Solitude's Radiant Raiment often times have Amulets, sometimes among them Akatosh. Otherwise, random drop. I can find Arkay's amulets all over the place in set places, but not Akatosh's.

Go to Radiant Raiment, check if they have it. If they don't, do not buy anything else, just reload the autosave from when you entered and check again, the inventory resets. If you don't have an autosave from disabling them, you can try leaving and coming back in, the inventory may or may not have reset though.I got my Amulet of Akatosh there, just took a few times and visits.

They did a pretty good drop with random loot from what I can tell (streaks are a sign of a GOOD RNG). I'm level 19 and I've got one of those and two of another one. Got the Dibella one which is the only one I really care about, but I'm keeping the others in case I ever get the urge to make the other shrines.