Definitely get new boots! You can have enough problems with shoes or boots that fit well; setting off in small boots almost guarantees disaster. If money is tight, sell the small ones on Craig's List. Since they're new, you should be able to recover at least half of the cost. I've had great luck on Craig's List.
Actually, before you do that, I'd go back to the store and explain the situation. If it's locally-owned, or the owner is savvy, they'll be willing to take the boots back and get you the size you need. If they do say no, offer to pay a small 'restocking' fee or ask for a nice discount on the new ones. I know a bunch of athletic footwear stores in my city that tell you from the start they want your shoes to fit, and if you run in them a few times and they just don't fit, bring them back and they'll exchange them, no questions asked. Plus, in this economy, a lot more stores are bargaining with customers.
Melanie