Vern Gambetta also sells one on his site but it is quite expensive and I’m not sure if the range of the speaker would work.
I can definitely sympathize with you though. I did the same thing you’re doing while I was working with sprinters at USMA and it made it very difficult to watch the runs. Also, figuring out when they were on pace or not on pace (and yelling accordingly) stretched the limits of my multi-tasking abilities (watching runs, timing runs, calling pace, figuring out slow or fast).
I also looked in to making my own device. If you have a little bit of electrical know-how you could make one yourself from stuff you could pick up at Radio Shack for about $50. The only reason I didn’t do that was because it would be a little bulkier than I had wanted.