Can anyone tell me where is a good place to buy the large 18" and 20" half round cake pans. I was at a wedding this weekend and saw a great large cake. I figured out they were using the half round cake pans.

Wilton only has half round in 18" and it's currently out of stock:
http://www.wilton.com/store/site/product.cfm?id=3E30AE35-475A-BAC0-5B68F137E1F8854E&killnav=1

Check with your local cake supply store... they may have it in stock or can order it for you from their supplier.

http://www.countrykitchensa.com/catalog/mini.aspx?ShopId=38&CatId=536&SubCatId=979

Magic line, Fat Daddios, and Wilton's make them - Amazon is your go-to...

http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=16%22+half+round+cake+pan&x=0&y=0

I recommend the Magic Line, quality pan with nice sharp corners.

Jen