I always hit up BestBuy,
But here is a lil arning, back when XP first came out, I went over to Ebay.com to buy it. I found one for $49.99. The buyer claimed is was brand new, still sealed in box. He even had homemade picyures of it sealed up, and it looked legit. So I brought it. When it arrived, it was in a regualr cd case, and on top of all that, it was a SamSung CD-R disc, and to make things worst, it had no liscense key nor did the cd work. The cd was blank.