Roxie,
All depends on which topiary pins you are talking about.
There are some topiary pins which you can only get by trading with Cast Members (called lanyard series pins). Those you will have to hunt for and hope you find them before other collectors do.
If you are talking about the Sorcerer Mickey, Donald and Daisy, or Lady and Tramp F&G topiary pins, you need to be an Annual Passholder to buy them. The Sorcerer Mickey pin has been on sale since the F&G show opened, Donald and Daisy go on sale this weekend, and Lady and Tramp go on sale 5/24, if memory serves me correctly.
There was also a set of 3 pins (boxed set) of the topiary at the entrance to EPCOT, but it was an LE of 500 and probably sold out by now.