I'm going to be the minority here, so bear that in mind. I prefer non elastic, no donut side reins. That is how I was taught to use them (ala Paul Belasik and posse) and find that many of the common complaints about side reins (causes leaning or hovering behind the bit) don't happen with non elastic side reins.
a few thoughts on elastic and such.... Elastic wasn't invented until 1820... the SRS came about in 1572 looooong before the invention of elastic so it's safe to say the classical masters weren't using side reins with stretchy parts (there may be good reason for that ;-)
I ordered mine from calevo, but I just searched their site and they don't have them anymore. These are from Dressage Extensions
I like them with a rubber donut in the middle of the side rein and the rest in leather. This way, if the horse decides to play around and buck or anything, the rein has some "give" and won't hurt the horse's mouth.
As for recommendations on where to get them, I can't help you there as I'm on the other side of the globe
I find that the elastic inserts in most reins are so stiff that they might as well not be there. I tend to only use side reins to provide a potential contact for the horse to stretch into, rather than an imposed contact if that makes sense. This lessens the tendency to lean or duck. My current reins are made of elastic bungee cord, which I was skeptical about, but they are actually very stable and easier to care for than the leather version. They're also wonderfully adjustable, as they have a sliding strap rather than buckles. My only complaint is that their lack of weight doesn't provide the same contact that leather reins would.
There are good things about both kinds. The elastic ones are IMHO are often better on young, untrained horses, so if they buck or question the initial connection, there is some give and they hopefully won't panic and go "up and over". If however you feel them "ducking/leaning" on the connection, then as an educated rider you need to push them more forward from behind, just as you would do under saddle, so they get off the forehand again and rebalance themselves. A more trained horse who has learned to not question the connection can do perfectly well in the non elastic ones, or Vienne reins. Good Luck!