If you want to do it youself, HorizonWirelessOnline.com - Horizon Wireless, The World’s Cellular Repair Center ß
has them at a good price, and it was fairly easy to do (at least on my Curve). I just replaced it a couple weeks ago because mine was blown, rattly sounding. I am not sure if you can order the part straight from the site, but if you email or call them I know they have the part.
Follow the disassembly guide in the FAQ and you should be good to go 8100 Disassembly - BlackBerryFAQ
Check out the pic between steps 5 and 6, the rectangular white piece in the top left on the front plate (the left of the 3 parts) is the speaker, just pull it out, and you might have to clean up the old seal if it doesnt come out, then pop the new one in.