https://www.varusteleka.com/en/product/helsingin-olkain-button-braces/57004 For smart wear.

And

https://russianarmyshop.eu/gorka/1507-suspenders-for-gorka-pants.html

For practical wear. It's very easy to adapt any trouser to work with either of these, for the smart ones you just need two buttons either side and the back, for the practical ones you just need a loop either side.