If you are buying new issues, the best place to get them is the online USPS store:
https://store.usps.com/store/home. For some sheets you can buy just a block of four. Higher value stamps can be purchased as singles.
You are going to pay a significant premium from online sellers for recent US stamps. It depends on how much you value convenience especially when they are still on sale by USPS.
For example, Mystic Stamp sells the Snowy Day Christmas Block of 4 for $3.95. USPS still sells a full booklet of 20 for $10.
I usually just mount the entire booklet on a custom page.