The mix of drugs is quite specific, because there are requirements above & beyond that which a murderer would care about. The drugs must be highly lethal - because generally you don't want to execute someone and have them survive it - but they must also not cause suffering to the victim. An overdose of a 'normal' drug isn't considered to be good enough. For example , as someone whose had morphine injections in my time, they make me incredibly nauseous before they take effect, which would make for a pretty undignified execution. So your lethal drug can't cause pain or major muscle spams or death by any means that someone would be consciously aware of. For example, you couldn't use a lethal drug that worked by paralysing the autonomic nervous system, because death by not being able to make your lungs breath is a pretty horrible way to go. The drugs mix generally does include a sedative along with whatever the lethal agent is.
In fact there IIRC there are only two or three agents that are certified to be used in lethal injection executions and only two or three companies that have the rights to make each one, so the supply of them is pretty limited. And, of course, you don't want to have massive stockpiles of them because they're basically pretty much guaranteed to kill whoever you inject them with with no fuss, which is not something you want floating around the place where anyone can get it.
Even the companies that can make them have been winding down their production of them because they get a lot of bad publicity for doing so - those who oppose capital punishment basically see them as being an accessory to state murder and aren't shy of saying so in the media. It's not a particularly profitable business and they get a kicking for doing it, so it's no surprise they're all looking to get out of the game.
