The actual movie files may be stored within the iDVD project files though I am not exactly sure where.
Right/Ctrl click on the .dvdproj file and choose "Show Package Contents". Go into Content>Resorces>MPEGnobackup and the movie file should be in there as an mpeg file.
Once the first one is made you can duplicate that finished DVD using something like Burn.
iDVD will prompt you to burn another dvd once it's finished burning the first disc. If you're wanting to burn extra copies at a later date, don't burn the iDVD project to disc, but from the File Menu, choose the Save As Disc Image option. You can then burn dvds as required using Disc Utility, which you'll find in the Utilities Folder in your Appliocations Folder.
If a lot of your "customers" want to view the movie on a PC rather than a TV, then you could always just Export as A Movie from iMovie's Share menu. That will give you a movie file that you can just burn to a dvd as a data dvd(rather than a video dvd) and they should then be able to play it on a PC.
Then again you could have the best of both worlds. Export As A Movie from iMovie and then Share from iMovie to iDVD. Before you burn the dvd from iDVD, go into iDVD's Advanced Menu and click on Edit DVD ROM Contents. Now drag that exported movie into the ROM Contents window and then burn your dvd.
Your customers then have the option to play the dvd in a dvd player, or play the movie file on their PC by inserting the dvd, going into My Computer and clicking on the dvd icon. This will then display the ROM Contents for them to play the movie file.
You just need to remember that some PCs/DVD Players will play home made dvds, and some won't. If the dvds that you're using are DVD-R, try changing to DVD+R or vice versa.