You can use an old leather belt if you want! Seriously it will work just fine.
Or you can locate some untreated leather (that shiny waxy leather in dress belts and shoes is treated) and glue it to a block of wood
Or if you just want to be lazy like me, you can order them online.
Amazon has some. I got mine from Bark River knives LINK
I would highly recommend using one though. If you're already proficient with sharpening, a strop will make your knives that much sharper.