It is a pattern to buy, you can buy it at pretty much any quilt store (in person or online).
So far as prewashing, I don't ever prewash fabric, but when I made my purse I did because though I like the crinkle look in a quilt, not so much on a purse, so I would prewash your fabrics so you can feel good about washing the purse later if it gets dirty.
Rachel