I second Murph's Stamps, but another resource you may find helpful if you are trying to track down a specific stamp or Scott # is ChampionStamp.com. To search their catalogue you do need to know the stamp's Scott #, which you can usually find by searching for the stamp on ebay as a lot of the listings have the scott number included.
Oh, and your local coin & stamps shop is an awesome resource too!! They usually have bins of random stamps that they sell at face value. It's a fun outing if you just want to find some random beauties. I was brave and brought by two boys (4yrs & 2yrs) with me to one shop
....but they were so good & happily watched some Thomas the Tank Engine on my phone so I could browse to my heart's content. The shop owner was really nice too and I think he appreciated how much excitement his stamps brought me. Letter lover for life!!
Chantelle