You’ll get the Choice Specs by speaking to an NPC in the Celestic Town shop in the morning (between 4:00AM to 9:59AM).
This is the only way to acquire a pair of Choice Specs in Pokémon Platinum.
As for the item itself:
The Choice Specs are a Hold Item that boosts the holder’s Special Attack by 50%, but lock the Pokémon into using only one kind of move until it’s switched out.
Choice Specs Location (Step-by-Step)
Inside the Celestic Town shop is a man who will give a pair of Choice Specs if you speak to him during the morning.
This item is one of three types of Glasses items that you can obtain by talking to this NPC at different times of the day – the other items are the BlackGlasses (Day) and the Wise Glasses (Night).
For the NPC to give you the Choice Specs, the in-game time has to be between 4:00AM and 9:59AM when you speak to him.
If it is, visit the Celestic Town shop to get your item – here’s a step-by-step guide on how to locate it.
Start by first Flying to Celestic Town and head north to the right of the Pokémon Center.
Go all the way up until you see some houses, then turn westward at the old man NPC.
Move to the very western edge of town here, and the shop will be directly in the northwestern corner beside a lake.
Head inside and speak to the NPC wearing dark glasses with brown hair.
If your in-game clock is reading a time between 4:00AM and 9:59AM, the NPC will hand you the Choice Specs at no cost.
Note: This is the only pair of Choice Specs in the game.
Choice Specs Details and Uses
The Choice Specs are one of three Choice items in Platinum, with the others being the Choice Band and the Choice Scarf. Each item gives a 50% boost to a different stat—in the case of the Choice Specs, they boost the Special Attack stat.
But all of these items come with a secondary effect:
They lock the holder into using only the first move it selects in each battle, and this effect remains locked until the Pokémon is switched out of battle (or until the battle ends).
Although the Choice Specs don’t enjoy the same popularity as the Band and Scarf, it’s still a highly effective item, especially when held by a Pokémon that already has a formidable Special Attack stat.
Some common examples of Pokémon who work well with the Choice Specs are Jolteon, Kingdra, Heatran, Latios, and Rotom-W.
Switching Pokémon with U-Turn
Switching Pokémon in and out of battle is a big part of using Choice items, and the Choice Specs are inferior to the Choice Band in this regard.
A Choice Banded Pokémon can make the most out of the move U-Turn, which switches the user out after dealing damage, breaking the move-lock condition.
Although U-Turn is a physical move, so a Specs holder misses out on the extra 50% damage boost you’d get by using the Choice Band instead.