I wanna get max cd as an AP support without buying Ionian's or upgrading the gold regen item

It kinda depends on what you mean by AP support. If you want to be building mainly AP damage items, then Morello`s is likely your best bet for a cheap source of CDR. Twin shadows is another option.

If you mean just a magic damage support, but not nessecarily building damage, then you have quite a few options.

If facing a heavy AoE team: Locket. If the enemy team is clearing out your vision and making picks: Twin Shadows. If you want some splitpush: Banner If your team benifits and it works on your champ: Ardent Censor. If the enemy team has a specific CC you need to worry about: Crucible. Zeke`s Herald is (I think) the most gold efficient item in-game if your team uses the aura well. Frozen heart if the aura will be likely to impact the enemy team sufficiently.

If your feeling confident, Mejai`s soulstealer is a risky option which is cheap, and can give a lot of AP and CDR.

Get the 10% from masteries. 10% 10% 20% 20%

Edit: Pick one 10, pick one 20. Obviously.

I usually have trouble with too much cdr as Support, pretty much every support item has cdr in it.

Include frozen heart, there's 20% (as a support you're reducing the enemies attack speed by 15% and able to take some hits, and it gives mana, so I don't see it being bad on an ap support).

DFG could be an option, 10% cdr and the active increases magic damage received from ALL sources so if you have a strong caster on your team you'll be amplifying their damage by 20% against their target.

Not sure of your other options, there are cdr boots (20%) if all else fails.

^Spell theifs and Athenes or Morellos otherwise get Frozen heart if your facing all ad and your support

uhhh knowledge is power. learn your items. typical support items that isnt ionian is frozen heart, spirit visage, athene's grail, twin shadows. these 4 have cdr if you mix and match them, youll get max easily. however it should all come down to how well you adapt getting these items won't guarantee you a win if you can't adapt to your game.

I play karthus mid, and I enjoy 40% CDR with this build: Seraph's, RoA, Deathcap, Zhonya's, Void staff and MPen boots.

What? 0% CDR in build? Must be from runes and masteries then. Scaling quints + blues give +30% @18, 9/0/21 masteries give 10%. Note: the AP ramps up very very slowly though, even more slowly if you're supporting and not farming.