They are made by Wilton, and they come in different sizes. You wet them, and then wrap around your cake pans before baking.......you can reuse them over and over. They are found at most cake supply stores.....here is the infor from Wilton on them:
http://www.wilton.com/store/site/product.cfm?id=3E30D620-475A-BAC0-592117323818A1AA&fid=3E32B7B1-475A-BAC0-5A817EB8A1425F9A
HTH
Kerri 
If you search the forum you will find there are threads that tell how to make your own with wet towels. Other people use an upside down flour nail to cook the center at the same rate as the sides ( I just did this for the first time with a 12x18 pan and it worked really well). for a small dome, I just press the cake down with a hotpad on my hand as soon as it comes out of the oven. I don't have to cut the crowns off anymore to make my cakes level-no wasted cake-or should I say no more cake on my waist-'cause you know the crowns are the best part!!
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