All the retailers that carry the Pop! collectibles offer specific items that are only sold by each outlet. There is currently a George R.R. Martin pop at Barnes & Noble only. This is a B & N exclusive. There is a sticker on the front of the Pop box to indicate this status.
Retailers began exclusive packaging on things like Blu-rays a while back. To increase traffic they started to offer items you could not get online. The treasure hunt aspect of finding a desired item enabled retailers to get some folks away from just clicking their mouse at home.
The demand for certain releases across the collectibles landscape has been quite eye-opening. Last month Amazon offered pre-orders for the new Queen Pops. Now they are only offering third party sellers for them. And the prices can become insane.
If you are lucky to live in a place where there are lots of choice you can usually still find them at a reasonable price.
For exclusives you should check online to see what items are being released. When visiting retailers you can browse through to find their exclusives. Always look for the sticker that says Exclusive.
My local B & N had a Hermione Granger Pop that was an Exclusive from a Funko Pop Convention! The price was $14.95. Cool item.
FYE will have an exclusive Freddie Mercury Pop! A diamond glitter pop styled after the rock icon’s stage wear on Queen’s Rock N America Tour in the summer of 1982 in support of Hot Space. This turned out to be his final US appearances.
Hot Topic carried an exclusive Bellatrix Lestrange pop, a villain in the Harry Potter series.
Store to store; region to region. These factors decide what figures you will find in your local shops. My closest B & N loves Harry Potter, Game of Thrones, and the video game Fortnite. These are heavily carried.
Check out your local places first. Then branch out to see what you can find elsewhere.