Echo that, most North American snake bites don't inject much toxin. Rattlers evolved to eat rabbits and the like- humans have a lot more mass than a rabbit. Now, if you fall into something like a nest of water moccasins you're probably just plain out of luck, but that is becuase you are going to be bit a LOT.

Buying antivenom... You can buy anything, if you have the right paperwork and permits, but for Joe Average who doesn't own/breed/sell poisonous snakes it could be difficult. It certainly is NOT shelf stable, it has to be in cold storage or it tanks pretty quickly. Even if stored properly, it has a shelf life. AFAIK, when it goes bad it just becomes inert, but don't quote me on that. I remember reading that anaphylaxis is not uncommon in cases where antivenom is applied, so...

And while rattlesnake venom is pretty much the same, no matter what bit you, coral snake is very different. Not sure on copperheads and cottonmouths, they are related vipers, and thus to rattlers. Then you've got spiders (sorry, nothing for the recluse) and scorpions. There is no universal shot.