I 'm trying to download QuickTimeMPEG2.dmg and cannot find a source. How can I download this installer?

So there is no way to get this .dmg?

If did not purchase a copy of this component from Apple, download and save a copy of the file, then there is probably no legal way to get it now since Apple no longer provides long-term access to purchase backups.

Streamclip cannot access the component included within the OS and specifically needs the .dmg file.

MPEG Streamclip only needs the DMG file if you are trying to install the QT 7 compatible version of the purchased AppleMPEG2Codec.component which is stored within the DMG file. This is an older version of the QT X MPEG-2 component and is stored in a different location than the newer component. If you have a purchased copy of the old MPEG-2 Playback codec already installed on an older system, then that component can be moved to your current system for single-copy use.

From what you're saying it seems because I don't have this already, and because Apple no longer provide it and Streamclip requires this version of it, Streamclip is not a viable option to rip a DVD.

MPEG Streamclip is functionally equivalent to QT 7 Pro but without QT 7 Pro's track compositing features. Neither QT 7 Pro nor MPEG Streamclip are "actively" supported any longer nor were either designed as dedicated "ripping" software.

Any recommendations for DVD ripping software?

http://www.makemkv.com/

It's no longer available for download and the component is now included with Lion and higher.

Handbrake. Google it.

Not to be used on commercial DVD's.