I used two cheap, basic timers (the ones with only start and stop, so I wouldn't be accidentally hitting other buttons) that I had laying around in my classroom. Worked fine! One timer counted up, and I set the other to count down. I also used a manual tally counter that I found cheaply on Amazon.com to click off each gate as I passed through, to make sure I didn't miss any gates. Gave me a little peace of mind that I wasn't off-course
I used to just use stop watches, now I use a wrist eventing watch. Dover Saddlery occasionally put them on sale or check Amazon. If you look under eventing timers you will find more options. I highly recommend having a back up as well. I actually use a wrist timer on each wrist and have a stop watch attached to my vest....and I've still been down to one during marathon with either dead batteries or accidently hitting it off.