Do you mean the APS installer in the website module to install applications into websites or the ispconfig extensions in the System module? The APS installer gets its packaged from here: http://apsstandard.org/
There is only one repository available from apsstandard.org as far as I know, but they accept contributions accoding to their website. You will find also all information about package faormat etc. there, ispconfig uses currently the 1.1 package specification and repository.