No respectable tie company will sell you a matching pocket square, because it's considered a serious fashion faux pas.
Instead, choose one color in the tie (the main color, a border color or even a secondary tone), and pick a pocket square which has that same color -- it can even be a different shade of the color.
It's fine if the pattern or texture are completely different.
This way, your pairing will look intentional and refined, not childish.
Here are some examples:
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