The ones I got from Dover are almost too for most of my horses. Dover Saddlery | Dover's Easy-Adjust Side Reins With Elastic.
I got these a few years ago and could not make them work on any of my horses and had to send them back. They were very long. Dover Saddlery | Dover's Easy-Adjust Side Reins With Elastic.
You might want to measure your trainers when they're set at their longest length, and call some of the tack stores and ask them to do the same. I don't know if Dover will (or can), but I'm sure some of the smaller shops would be more than happy, like Adams Horse Supply or Dutchess Bridle & Saddle (they are a full shop, only listing on their web site some of their main items).