I'd suggest buying different sizes, and sending back the one that doesn't fit, which should be totally free for an Amazon Prime eligible product.

Some insoles also have cut marks so you can just adjust it to your needs.

Here are some examples after a quick search.

https://www.amazon.com/Sof-Sole-Memory-Plus-Cushioning/dp/B00PKM76U0/ref=sr_1_32?keywords=flat+insole&qid=1569316544&s=gateway&sr=8-32

https://www.amazon.com/Spenco-Comfort-Lightweight-Cushioning-Orthotic/dp/B000S6GBI6/ref=sr_1_4?keywords=flat+insole&qid=1569316544&s=gateway&sr=8-4

https://www.amazon.com/Cork-Insoles-Pack-Size-13/dp/B0043X90ME/ref=sr_1_5?keywords=thin+cork+insole+men&qid=1569316788&s=gateway&sr=8-5

https://www.amazon.com/Generation-Anti-Bacterial-Killing-Insoles-Inserts/dp/B07KFLGL41/ref=sr_1_25?keywords=kaps+insoles&qid=1569317227&s=gateway&sr=8-25

Personally I use Kaps Cork adjustable insoles but they are not sold on the US Amazon (I live in France), but they are very similar to the ones I linked last.

Lastly, you can also go to a Foot Locker or this kind of Sneakers shop and ask for a simple insole, this way you would have the opportunity to try them on and see if they fit. I have an old pair of AND1 flat insoles from when I used to play basketball a decade ago and they are still in a pretty good shape, I use them to replace my cross training shoes soles to flatten the drop.

I hope it helps.