Hi there.
You may or may not require a new chimney pot. Many can be salvaged and reused, although this is not always the case. If the pot is damaged, or you are unable to get the old mortar off completely, you may need to buy a new pot.
Good quality chimney pots generally start at about $300. It's worth dealing with a reputable dealer, as there are pots out there that have not been made from the proper materials, fired at the right temperatures, etc.
The last thing you want is to have it fall off the chimneytop - both expensive and dangerous. So you should deal with a "real" supplier that inspects all pots by hand - especially important if you are looking for an antique pot.
The world's largest supplier of chimney pots is actually in PA and will ship anywhere. The Chimney Pot Shoppe is online at
www.chimneypot.net or you can call them at 724-345-3601. They literally have hundreds of styles to choose from and stock antique and new. Custom pots are also available, but they get pricey.
Good luck.
If you are interested in installing the pot yourself (despite this being DIYnot.com!), they are very easy to install. The June issue of Journal of Light Construction will have a feature on installation of chimney pots. Or just ask the people at the chimney pot shoppe - they can tell you what to do.
Good luck!
Ola Lessard