Where to get textures to use in marketplace products?

Hi folks!
I’ve just become aware that we aren’t allowed to use content in marketplace releases (landscapes in my specific case). As such I was wondering where do you folks get the textures to use in the scenes or models that you sell on the marketplace.
I’m sure some of you create textures from scratch, but certainly not all, so I’m curious to know if you are aware of texture sites whose license allows their stuff to be used on UE4 marketplace products.
Any feedback you can give on this matter is truly appreciated.