The clicker I use (and love!):http://www.amazon
The must-have wrist attachment (I reccommend a more expensive and high-quality one so it doesn't break with all the things you do with horses, including riding. Mine's lasted over a year): Amazon.com: Customer Reviews: Securit Wrist Key Coil Wearable Key Organizer, Flexible Coil, Black (04995)
The book I HIGHLY reccommend for starting clicker training with horses and refer to often: http://www.amazon.com/The-Click-That-Teaches-Step-By-Step/dp/0970406509/ref=sr_1_1?ie=UTF8&qid=1359347309&sr=8-1&keywords=the+click+that+teaches
Now, all you need is a reinforcer (aka reward) that your horse will work for, and you're good to go!