The ACLU is offering a FREE pocket size copy of the US Constitution. Just enter coupon code POCKETRIGHTS at checkout. It does say it's backordered, but since it says it's available free until election day, I'd assume it will probably come back in stock.
If you just want a copy for yourself, you can pick one up for $1 at Amazon. Prime shipping is available or you can download the Kindle version for FREE as well.