Steve M,
I do all of my reaction training during dry fire practice in my home. If you have a timer, I suppose it would help but I find that using odd sounds or commands makes the situation more realistic. For weapon presentation, I usually have someone give me a signal to draw and then engage a target. Moving around and not standing in a ready position helps. Start from different positions such as kneeling, lying down, punching, backing up, rolling, etc. Also, use music in the background, act like you're washing your car or visiting the ATM, etc. Don't just stand ready and waiting for the command.
Repeat over and over.