The trackpad itself is part of the topcase assembly and can't be replaced without replacing the entire topcase. Underneath the keyboard is the trackpad connector, and the first step should be to disconnect the connector, re-connect it, and see if that helps. There are different connectors for the different models, but the concept is the same, and you can find instructions for disconnecting the trackpad connector in the guides for replacing the topcase on this site. Please note, you don't have to take off the entire topcase in order to disconnect and re-connect the trackpad connector -- you just need to remove the keyboard, Airport (in most cases), and memory cover.
Contact cleaner will most likely not help.
It's possible you had a bad topcase and will need to replace it, or that the socket on the board is coming loose, in which case it's not likely a fixable problem.
Let me know what speed iBook G4 you have, and whether it's 12" or 14" -- the 1.33GHZ 12" and 1.42GHZ 14" have multi-touch trackpads, and sometimes behave as you describe with variations of Tiger or older operating systems, so if you have one of those machines and one of those operating systems, it's conceivable that it's a software issue. Did it ever work well, or has this computer always had the problem you describe?