so I went for a few months without a binder, ended up caving in and buying one bc they work reeeeally well. but before I got one I had some tight-fitting sports bras that I would layer on until my chest was as flat as i could get it. I even made them a bit tighter with safety pins but you have to be really careful cause they could suddenly open up. it won't be totally flat (at least, not as flat as a binder could make it) but this method worked okay for me.
I would advise against using an ace bandage since they're made to restrict movement. also don't bind for more than 6-8 hours at a time and also take breaks twice a week!
If you're wondering I got my binder from gc2b and it was a little pricey but totally worth the investment