[luv-beginners] where does Ubuntu software centre store its packages?

Hello, I'm using Ubuntu 11.04 and have been trying to get the GnuCash package onto CD.I tried APTonCD but couldn't find the relevant package with it.I assume its just a matter of finding the package on my hard drive if it even exists.GnuCash is definitely installed.I tried googling for about 15 minutes and also the inbuilt Ubuntu help which is not working properly for me. regards Peter