‘Fraid it’s nearly impossible to find a copy of LR standalone any more. 🙁 Not likely too many people want to part with theirs if they do have a copy.
Just for fun, I did a search, and did locate a “brand new” copy of LR 5 for $69. No guarantees that it will work though; the seller is a brand new one, so doesn’t have much of a reputation yet. Proceed at your own risk. (Personally I’d be leery of it, but there’s always a chance it’s legit.) Here’s another more expensive option that might be better.
If you do track down a copy, make sure you backup the installer and the serial #, cause you can’t even download them any more from Adobe.
I also checked into LR alternatives that can import info from LR such as tags, collections, edits, etc. Darktable claims to be able to import most of the basics from LR, but it definitely won’t be 100%, especially for actual photo edits. (They’ll look different because of different algorithms between the software). So that’s something to consider if you don’t want to spend money trying to keep LR afloat for the rest of your life.