ORIGINAL: Urban Legend
B and H is a great place. You may be able to price shop and find cheaper, but to me its not worth it. They usually have everything in stock, ship fast, and have great customer service.
http://www.bhphotovideo.com/
X2, but if you're on a budget check Ebay. There are a lot of good used tripods for sale there. You can usually find a good number of the 501s there as well.