Personally, any CD besides Platnum (one or two), the Best of Elmo, Kid's favorite songs, and Songs from the Street, are tricky to find. I remember I had to special order Born to Add.
With the exception of Songs from the Street, those albums ar ethe ones that I most commonly see in stores these days. I have also seen Hot! Hot! Hot! Dance Songs in a few different stores a few years ago. Back in 2004, there were two new Sesame Street CD releases, A Celbration of Me, Grover! and Happy Birthday from Sesame Street, and I've only seen those in a few stores on a few occassions.
It seems like in the last few years it has also been harder to find certain Sesame Street DVDs. Back in 2004 it was easy to find most of the DVDs that were available, though many stores only sold certain Sesame Street DVDs, and now it seems like more and more are becomming harder to find, even in stores that sell big selections of Sesame Street DVDs, like Toys R Us. A few years ago, Wal-Mart sold a lot of Sesame Street DVDs, but now it seems like Wal-Mart only sells a select few, and mostly recent DVDs.
I wonder if any of the DVDs are out of print, or if they are just too hard to find. I could see if Genius wanted to get older ones sold and then rerelease the older DVDs, but I believe that it started to get harder to find certain DVDs before Genius got the DVD rights to Sesame Street.