Depends upon how you want them to fucntion. If you want them to turn on with the stock footwell lights, then you can tap into the existing wires running to that. To minimize the added load, I'd convert the stock light to LED first.
If you're looking for them to turn on all the time at night (i.e. mood lighting), then you'd need to tap into something that turns on with the parking or headlights. A relay would be the safest, but with the relatively low current draw of an LED strip (assuming this is what you're looking to do), tapping directly to a source is probably OK.