Almost all of the BSD releases have been well preserved. If you want to find 1BSD, or 2BSD or 4.3-TAHOE BSD you can find them online with little fuss. However, if you search for 2.11BSD, you’ll find it easily enough, but it won’t be the original. You’ll find either the latest patched version (2.11BSD pl 469), or one of the earlier popular version (pl 430 is popular). You can even find the RetroBSD project which used 2.11BSD as a starting point to create systems for tiny mips-based PIC controllers. You’ll find every single patch that’s been issued for the system.
What you will not find, however, is the original 2.11BSD release tapes. You won’t find the original sources. With some digging, you can find is 2.11BSD pl 195. This was released about 30 months after the original was released, and is the oldest one that’s known to exist.
And so starts the search for the original code.