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You can get them in the cash shop, or you can buy airtight ones in the marketplace. You can also buy some from the AP shop (talk to Ernmass in the Colhen Inn)
I remember the old Nightmare at the Ruins AP farm from forever ago, but I'd recommend either getting up to S2 raids (probably level 80), or if you want more AP as you level, battles tend to give diminishing AP as you battle them more and more, so you could repeat battles you've done before until they no longer give enough AP that you find worth it.
If you do the S2 raids, you can pick an choose, or do them all. You can run them 6 times per day, for this event (or 7 if you have VIP). Running each of them 6 times a day would net you 3,000 AP.
If you have VIP (you can find VVIP capsules on the market, or you can buy a VIP 2 day capsule from Ernmass in the Colhen Inn) then you could make 5,250 AP per day. I think the VIP capsule is either 500 or 1000 AP, so you could make it back quickly.
I think I saw you mention in a previous post that you're an Arisha (same here! yaaay!) For her to have all her skills maxed costs about 108k AP if I recall.