What is a good insertable shoe insole for flat feet?

Variety. There are many good otc insoles. A few of my favorites are: powersteps, superfeet, spenco orthotics and birkenstock. There are many others. You need to try them on as each will feel a little different. Too much support may not feel comfortable if your foot is very flat. Dr l.
Depends if. You have a flexible or rigid flatfoot. Ideally, I believe a custom device is the best as I often find a person with a flatfoot has a functional hallux limitis and a custom device is designed to treat your particular foot. If cost is an issue you can try brands like powerstep, lynco, superfeet.