Good place to buy poison - every village has an apothacary shop. This is also where you would go to buy app 2 versions of spells if you wanted them. In that shop will be an NPC who sells poisons and other potions.
In South Qeynos, there is also an apothicary shop, (listed on your in-game map), with an even larger selection of poisons. (Be sure and check upstairs there too.)
And finally, you can usually find the best poisons for any level on the local broker. Player made poisons have more charges per vial and a better selection of effects. But they also cost a lot more so if you have a friend ingame who is an alchemist, he can probably hook you up with any poison you might ever want to try and save you some money.
Poisons come with a large variety of different effects. Some cause assending or desending damage with some combination of small medium or large initial effect, (front end), followed by small medium or large Damage over Time effect or DoT, (back end). There is always a trade off, so if you want a large DoT it will come at the cost of a small front end and vice versa.
In addition, there are poisons that de-buff the mob to one or more kinds of damage. These can be extremely useful when you are in a group with a player who can cause the type of damage you are de-buffing.
You can apply multiple poisons as long as they are of diferent types. So, you could use a desending damage poison and a strength de-buff at the same time.
And finally, since it may well be your next question... to use a poison, place it in one of your two activatable slots, then right click on it and choose "use". An application of poison will last 12 hours or 100 procs - whichever comes first.