There are a few easy things you can do to increase your wrist strength. The classic "roller" where you use a piece of rope attached to a broomstick or dowel, which is tied to a weight is probably the easiest and most common. I wouldn't suggest buying a premade version, because I think you can make your own, customized to your hand size, at least as cheaply. Wringing a towel is a similar exercise, as well.
You can also use a staff or pole; hold it near one end, and try to keep it straight and level. Once you can simply hold it, you can start moving exercises with it.