Premium Aquatics (http://www.premiumaquatics.com/) is where I obtain my frozen Mysis. They offer the usual jumbos as well as the small Piscine Energetics frozen Mysis. Their supplies are limited, but when they have them in stock, that’s where I get my smalls. When they’re out and I want small frozen Mysis, then I pick up some Hikari frozen Mysis from my LFS instead. They are the smallest of all the frozen Mysis and are usually well accepted by young seahorses.
And, of course, you can also cut the king-sized PE Mysis into bite-size pieces for your small Mustangs as well. It’s a little more trouble, it usually works fine in a pinch. Try target feeding the chopped Mysis to your ponies using a turkey baster, if you’re having trouble locating naturally small frozen Mysis.
Best of luck feeding your Mustangs, sir!