I have to agree with Prons, I get most of my theme decks from Amazon because of the ease of it, but I tend to stray away from booster packs for some reason. If I'm going to get some booster packs, I get a booster box, and purchase it off eBay from someone who has 100% Satisfaction User Rating.
If I'm looking for individual cards, I tend to look at professor-Oak's website. I've never had any problems from there, and while the price may be slightly higher, since I've never had any problems, I don't worry to much about it.
GibbyGibson