I use one from Rick Steves, the sort that attaches to your belt and gets stuffed down your pants.

https://store.ricksteves.com/shop/p/hidden-pocket

I often don't use it as my travel generally doesn't take me to places infested with pickpockets. If I'm in a city like Tokyo or on a business trip I don't bother.

However, I'm on my way to Vietnam next week so I should probably dig it out of my closet. In cities where I am worried about pickpockets I'll keep my passport, credit and debit cards in my money belt and just carry some cash in my pocket. Enough to get me through a day or two but not so much that I'll care that much if it's stolen.