Cutting your own FQ from a yard is probably easiest, but it seems like a shame to make those cuts if you will want a larger piece for something else later (unless I'm not understanding this). Even if you just slice out an 18x21 rectangle, you may be left with weird, small pieces for your scrap bag. Better, if you don't mind all the number crunching, to figure out what will really be needed and cut just that.
Hugs,
Charlotte