I love using hydrangeas for these carriage as the provide a big pop of color. You'll want the type found in a greenhouse (hothouse grown).
The 4"-6" pot usually does the trick. And if you have room add some ferns, foliage and ivy.
Then you'll want to secure the wheels with ribbon or raffia. It dresses it up a bit, while providing more stability.
Then a pretty bow on the handle!
The wire carriages can be found locally at Leaf and Petal in Mountain Brook or in Atlanta at Boxwoods.
Let me know if you are interested in me making one if you are here locally. I've also put it on a grapevine wreath before and it is super cute. Great baby shower table arrangements too!