Sandhill Preservation has tons of information about growing sweet potatoes and especially about getting them to grow in northern regions and common misconceptions. Check out this link:
Some of the misconceptions are (hit the link for a lot more details):
First: Forget what climate zone you are in.
Second: Do not set the plants out when it is very cool.
Third: You do not need lots of roots on the slips when you plant them to be able to insure success.
Fourth: This is the most important thing when it comes to sweet potatoes. It is the heat units that determines success, not the number of days nor plant zone, but heat units.
They also have an insane number of varieties of sweet potatoes: 225 VARIETIES!!!
Here's the link for the catalog and shipping info:
They offer discounts if you get them shipped after June 25th too.
One more thing. If you don't already know it, Sandhill Preservation is an old fashioned company. You can't order online. You have to print out the order form and mail it in with a check. I actually sort of enjoy the quaint time warp quality of ordering from them.