I don't believe you'd need to do anything special to have plate that accommodates your wings. Here's my reasoning:
In reality, you don't just buy a set of plate (or most armor) off the rack; It has to be crafted to fit you. My assumption as a DM and player in D&D has always been that when I go to the blacksmith to buy my plate I'm putting in an order with my specifications and getting back what I asked for. Following this line of thinking, it's safe to assume that you can simply specify your armor is designed to accommodate your wings at time of purchase.
Additionally, there's even note in the Armor section of the PHB under VARIANT: EQUIPMENT SIZES regarding this line of thought:
In most campaigns, you can use or wear any equipment that you find on your adventures, within the bounds of common sense. For example, a burly half-orc won’t fit in a halfling’s leather armor, and a gnome would be swallowed up in a cloud giant’s elegant robe.
The DM can impose more realism. For example, a suit of plate armor made for one human might not fit another one without significant alterations, and a guard’s uniform might be visibly ill-fitting when an adventurer tries to wear it as a disguise.
Using this variant, when adventurers find armor, clothing, and similar items that are made to be worn, they might need to visit an armorsmith, tailor, leatherworker, or similar expert to make the item wearable. The cost for such work varies from 10 to 40 percent of the market price of the item. The DM can either roll 1d4 x 10 or determine the increase in cost based on the extent of the alterations required.
I wanted to be sure I wasn't out of my mind so I checked with the league admins and they said that, unless there's something in the item's description that would prohibit it from accommodating wings, my logic holds true:
I asked Alan Patrick, Resource Manager for Adventurers League and Greg Marks, Content Manager for Adventurers League
@gksmithlcw: is there anything in AL that precludes someone who purchases plate from stating at time of purchase that it will accommodate wings (for, say, a winged dragon sorc)? Thanks in advance!
@warfteiner [Alan Patrick]: Not unless there's verbiage about that in the item's description (PHB/source for mundane items, DMG/adventure for magic).
@skerrit7h3green [Greg Marks]: no
So, there you have it. Just state that it accommodates your wings when you purchase it and you're good to go.