Zavvi has a bunch of them usually on sale
https://us.zavvi.com/elysium.search?search=reaction
Other than that I've seen them on amazon and most of the site sponsors carry some of them.
I've only picked up a couple of these as they tend to be more expensive than the other retro figure lines. Only Transformers one I have is the Rumble/Frenzy (whichever one you think is the blue one) as a decent G1 accurate stand-in to go with Netflix Soundwave.
