For all of the wrenches, I made them out of pine wood. Nothing fancy, just as long as it has no problems with it. You might even want to upgrade to select, with no notches or blemishes. Pine is cheap, abundant, and quite forgiving. It's also really easy to cut (comparatively) to other woods. It's also flexible, so you might be able to drop it on the floor a few times before it breaks.
Right now, I'm using cherry wood on everything. It's a nice hardwood that looks great and isn't too hard to work with. However, its hardness makes it rather brittle, but its strength is still well above pine wood.
Basically, what I'm trying to say is, if you've done woodworking before, go ahead and use something a hardwood like cherry. If you're new to woodworking, or need to get the project done cheaply, a nice 2" x 6" select piece of pine from a hardware store will fulfill your needs.