Need a dragonite counter... pls help.

cloyster with avalanche is a good option potion-wise

Cloyster with frost breath/avalanche is usually a strong and cheap Dragonite counter.
Weavile is another very good option if you have a Sinnoh stone

I used to fight it with my L33 Mew, but that's about to end now that I got Weavile. I'd need to test it, but I believe Ice Beam Mewtwo is enough even without powering it up at L20.

For mons easier to get, Cloyster, Piloswine and Jynx do a decent job.

Any ice pokemon at level 30 should do the job pretty fast. I'm surprised that you struggle with walrein. As many said cloyster is probably one of the cheapest options, if you catch a high level in the wild evolve for double ice moves.

I'm going to approach this question from a different POV: given that you have serviceable counters (and you do), why are bog standard healing items too expensive for you to spend regularly?

Slowbro with water gun and ice beam.

Nothing beats Ice for taking down Dragonites. Personally I prefer Piloswine, the combination of Avalanche with the high energy gain of Powder Snow, if played right, is fast enough to take out a Dragonite defender before he can fire his first charge move.

But if you want to save potions, go for bulk. A legacy Lapras with ice Beam is great.

Standard gym counters for Dragonite are anything with Ice attacks to take advantage of the double weakness. Given the latest shift in weakness/resistance values, this is more important than ever.

Jynx, Walrein, Cloyster, and Lapras are standard counters. Once it's out (maybe in a few days) Mamoswine will excel in that role, with Avalanche. Further, it will have CP comparable to Dragonite's CP (once you level up), and get STAB with it's Ice attacks (unlike Empoleon).

Mewtwo with Ice Beam is also good, but I wouldn't recommend investing RC in that now, with Mamoswine imminent [possible release Tuesday (tomorrow as I write this) at 1 PM PST].

Milotic is avesome against dragon!

There isn't much I can say that hasn't already been said. Ice types are the fastest way to take down a Dragonite especially with the recent change to effectiveness multipliers. Preferably you'd want to use something with either Ice Beam or Avalanche as their 2 bar nature can help you get a jump on them if Outrage is giving you grief. Weavile has the highest DPS - if you have a Sinnoh Stone it's worth looking into. For potion effectiveness Cloyster or Articuno would be better picks.

I'm surprised Potions are a limit for you. Do you open 20 gifts daily? I find I have to chuck healing items just to make room.

tl;dr : If the problem you're facing is potion-usage against a Dragonite, your only real option is to Dodge.

Full Explanation:

Every Pokémon that will have a chance of taking down a Dragonite quickly is going to have more than 115hp, meaning when you revive it, it's going to need at least 58hp - which is either 1 Hyper/Full or 2 other potions. And if your Pokémon doesn't faint, you will likely need 2+ Potions (other than Hyper/Full) to bring it back to full health.

A CP 3500 Dragonite is going to cause a lot of damage when it hits your Pokémon - no matter what Pokémon it is. And if it has Outrage, it's going to get at least 1 of those off per Gym Defense - Dragonite, even with the double-weakness to Ice, is just that strong. So it's going to take down any and every good counter for it

The only other potential option is something like Gardevoir with Dazzling Gleam. Fairy does Super-Effective damage to Dragon while taking double-reduced damage to Dragon (Assuming that the Dragonite doesn't have Steel Wing as a Quick Move, in which case Gardevoir is terrible). But even with Gardevoir, you're going to need to dodge to avoid the damage if your concern is conserving Potions because Fairy is only single-Effective against Dragonite, not Double-Effective. So less damage taken per hit, but will take more hits per battle.

So, in summation, if you want to conserve Potions - DODGE. Dragonite is a costly defender.

(and if you're having trouble getting potions, now is a very good time to get a pretty big Friends List going and trying to get about 20-30 gifts coming in a day. My gifts tend to contain about 20% Potions of one kind or another, and new Baby Pokémon are coming to 7km eggs. Hopefully these Baby Pokémon will still have the 25 candy/10,000stardust cost to buy a new Charge Attack, so it might be possible to have a Snorlax with 2 Charge Attacks, for instance, instead of having to pay 75 candies and 75,000 stardust... I know I'd be interested in that, if only to have something unexpected to toss out in PvP.)

It sounds like a big part of your issue isn't the ability to take down Dragonites in gyms, but rather the number of healing items you have to use while doing it. Thus, I'll recommend Togekiss (I don't think I saw anyone mention it yet). It won't take down a Dragonite with the speed of an ice-type Mon, that's for sure, but with immunity-tier resistance to Dragon-type attacks, your Togekiss should need little to no healing after a fight with a dragon.

Meteor Mash Metagross, Togekiss, Your own Dragons and Rhyperior works if you don't want to waste for ice types. Dodging required however.

Dragon Tail/Stone Edge Aggron- hes slow compared to rest, but resist everything Dragonite can use to you, and with his huge bulk wont drain many potions.

Winter (event) is coming! Featuring loads of ice type pokemon, so your problem will likely solve itself in the form of a high level Jynx or Piloswine. And if by any chance they release Mamoswine (now would be a good time), Dragonites won't be sitting in gyms for long.