Your local hardware store might have what you're looking for. Call ahead before making your purchase just to make sure that they do. If not, try searching online for where else you can buy one. You'll likely be able to find one with relative ease – they are very popular among chefs everywhere along with the word-famous chef knives.
Whetstones vary in price depending on how big they are, and where you get them from – so be sure to check around before purchasing one. Having a whetstone handy will help with more than just sharpening your serrated blades though – you can use it on any of your kitchen knives.