Need: Adventure Involving Counterfeit Currency

You can also read up on historical counterfeiting schemes; regardless of the system, I like using bits of real-world history to ground them in.

I don't know of any content exactly as you want it but ...

There is some content in Fizban's related to cursed coins from a Dragon Hoard.

or

Mold earth to make a sand mold of local silver coins joined by a small sprig.
Pour iron into the mold and let it cool.
iron to silver with Minor alchemy via 2nd level Transmutation ability. (one object only)
Prestidigitation to make it look appropriately weathered. (one object only)
Break them into individual coins.
Pawn them off in an hour
Profit?

Adventurers paid by crummy minor noble to determine how the counterfeiting was done and track down the culprit.

Use this site to generate the city if needed: https://watabou.github.io/

Clues: Smithy or tool proficiency to determine sand casting using extremely fine sand (rare). Arcanna check to determine Transmutation. Nature or local druid to find deposit of such fine sand. Stakeout to catch lackeys digging up sand for another batch. Lackeys lead to an abandoned bakery on the outskirts of the city, the Transmuter's lair. Transmuter, mephit familiar, bandit type lackeys. Maybe a group of dwarven anarchists.

Plot twist: Transmuter is a philanthropist. The poor finally were able to attend the city carnival. Back taxes paid. Do they let her go if she promises to stop? Do they turn her in for the reward? Do they join her?

Battlemap for a bakery further down the page.
https://crossheadstudios.com/battlemaps/

I haven't looked at it yet, but maybe Keys from the Golden Vault has something you could use for inspiration? Seems like the adventures therein are meant to be 'heist-y.'

In general, keep the following in mind when making an adventure:

• WHAT? What is the premise of the adventure? In this case, it the basis for your post: "Counterfeit currency is being circulated through the local economy."

• WHO? Who is/are the primary actors in the situation? In this case, WHO is circulating the currency? Are they in a traditional position of power (government, community leader, faith leader) or are they a small fry or underdog (peasant, small organization, charlatan)?

• WHERE? The setting and context matters. In this case, is "the local economy" a kingdom, a single city, a small town...? This can define the scope of your adventure and give context as to why the issue needs adventurers to resolve it. A city's treasury might not have the manpower to mount an investigation, a village might not understand why it's a problem, or maybe even largely not care ("it's only a problem for the merchants"), a kingdom might be dealing with a different or related financial crisis.

• WHY? What precipitated this issue? Is someone just trying to get rich? Is it part of a greater political maneuver to frame or otherwise disgrace someone? Is this setup being used to fund some greater scheme (set up a longer plot)?

• HOW? How is the plot being accomplished by the big bad? You don't generally need to go into intricate detail. The rule is generally "go two layers deep" to make it seem like everything is intricately thought out. "The antagonist is actively spending fake currency around the city to put it into circulation. Why can't the merchants tell it's fake? The core of the coins is an amalgam whose weight matches gold." "The antagonist is replacing coins in the treasury with fake coins. Why doesn't anyone know this is happening? The antagonist found a secret passage on an ancient map." Etc