Spider mites can be found pretty much anywhere that its hot and dry. They can be in your grass, on your pets, on your clothing, etc, and chances are you won't know it until you start to see the signs that they've been eating your plants. They prefer to hang out on the shaded underside of leaves, and thats also where they lay their eggs. There are a ton of different ways to deal with them, but I'm partial to a combination of once a week neem oil sprayings and having a hot shot no pest strip hanging in the middle of my pots. You just have to make sure you shut down your fans and ventilation for a few hours during the dark period so that the hot shot will work properly (it needs still air to function).
If you have an active mite infestation you aren't usually going to get rid of it with one treatment. Neem oil keeps the adults from reproducing and kills eggs that it contacts directly, but it doesn't work over night. You will usually have to spray several times to completely get rid of the infestation, and since you don't want to spray more then once every 5-7 days you need to be patient. Spraying more often can cause the neem to build up and block the plants ability to process various things, which results in stunted growth. You also need to remember to spray the plants with fresh water the day after using the neem to wash as much of it off as possible. The particular brand of the stuff i use is called Einstein oil, and its about $10 for a bottle that should last you quite a while if your only spraying a dozen or so plants once a week. Just go easy on the stuff- I use the lowest strength (.5 tsp per qrt) for my weekly preventative spraying, and never more then 1.5 tsp per qrt to deal with an active infestation.
