iDVD was never offered in the App Store (FYI the iTunes store is for apps for mobile devices only).
iDVD was only available via iLife discs, iLife 08, 09 and 11. Since iDVD was discontinued about 5 years ago the only place to get a disk is from a 3rd party vendor like Amazon or eBay. If you plan on obtaining iDVD be sure to get the iLife 09 disc as it's the only one that included all of the themes available for iDVD. Be aware that iDVD is marginally compatible with Sierra and High Sierra. It will work but with some difficulty and is somewhat expensive. The iLife disk will install iDVD 7.0.1 and you can upgrade to the last available version, 7.4, via the updaters at the Apple Download site.
However, you would be best served to look for an alternative. For burning just movie files to video DVDs Toast is a good choice. For both movie files and still photos for a slideshow then the only alternative I've found is DVD Maker Pro-DVD Creator Burn from the App Store. There are two versions, a free demo version that adds a watermark to finished projects and the full version for $15. Try the demo version to see if it will do what you need before purchasing the full version.