I provide the standard box clicker on my site. You can order a set of three for $7.50 plus shipping. These days many pet stores also carry clickers. We use the same clickers for dogs and other animals that we use for horses, so if all you need are clickers, you can save yourself some postage by buying them locally.
There are now several different types of clickers available. The box clicker that I sell was the original type. It's still the one I prefer. The clickers I provide all have tabs attached so you can attach them to a wrist band for easy access. The box clicker makes a somewhat sharper sound than the newer I-Click, so it is more suitable for the kinds of environments we typically work with horses in.
If you also work with dogs, especially with fearful dogs, you might prefer the I-Click. They are easier to click than the box clicker which can be an advantage. But it also means with horses they are easier to click accidentally. You can order I-clicks from Karen Pryor's Clicker Training site: clicker training.com
There are also several electronic clickers on the market.
If you are new to clicker training, I don't want you to get hung up on the tools. I use the box clicker for the first couple of sessions I do with a horse, then I switch over to a tongue click. The box clicker makes a sharp, quick, distinctive sound so it's a perfect marker signal. Using it as you get started helps your horse catch on to the significance of the sound faster. But for working with horses I want my hands free, so I quickly switch over to a tongue click. That's so much better. The horses have no trouble making the connection, and I never have to worry about where I left my clicker!
Another quick comment: clicker training is information driven. Getting a clicker without also getting the information that explains how to use it won't get you very far in your training. In fact it might even get you into trouble. That marker signal is incredibly powerful. Horses catch on fast and they become very eager to play the clicker game. I've spent the past twenty plus years working out this training program, looking at what works, what belongs in the beginning steps, what is better left until later, and what are the handling skills that help make the transition into clicker training easy and successful.
If you are starting out and you are also ordering books and DVDs, it makes sense to add in a set of clickers. They don't add anything to the overall shipping cost, and you'll have everything you need to get started. But if you are thinking about just getting the clickers and skipping the information on how to use them, I would say you have the cart but no horse. And in this case the information is what drives the process forward to great things.