Here are a couple of sources for chimney mounting kits:
http://www.solidsignal.com/cview.asp...ing%20Supplies
http://www.3starinc.com/roof_mounts.html
If you have a hard time locating 10' TV antenna mast... I've used 1-3/8" diameter chain-link-fence top rail from Home Depot with success. The 10' mast can accommodate two antennas. A 5' mast section would have enough room for a single antenna, but on a chimney, you would want to mount higher, to keep the antenna out of the smoke - to avoid corrosion.
If you have no conflict with other viewers over which way to aim, I would suggest you add a rotator... You'll have the option of aiming at all markets rather than being forced to choose only one aim point. Consider the Hy-Gain AR-40, if that is too expensive, then consider the CM-9521A or NTE U-106 (either of which would benefit from the addition of an NTE TB-105 thrust bearing. One other aside re. rotators; you'll want to be sure your TV can scan for new channels without forgetting past scans... It can be frustrating to use a rotator if your TV has to fully rescan each time you turn the antenna.
http://www.hy-gain.com/Product.php?productid=AR-40
http://www.3starinc.com/antenna_rotors.html
http://www.solidsignal.com/cview.asp...nna%20Rotators
Given the weaker signals, you'll want to consider using a preamp such as the Antennacraft 10G201.
http://www.antennacraft.net/Antennas...mplifiers.html
(Shop around, many of the products can be found in other retail outlets... I've offered the links as a way to be sure you're looking at the product I'm referring too.)