I want to build my own 4 point bending tester. First, I need to see what parts are available for sale. Can anyone tell me if the circled L-shaped part has a name I can use to search with, and if so what? Alternately, any suggestions as to parts I could use in a similar fashion would be useful.
The part you've circled is typically referred to as an "anvil", or sometimes a "loading strut" Two are usually secured at the bottom, and either one or two anvils are used on top (depending on a 3 or 4 point bend test).
I would certainly take a look at products made by Instron: Instron General Purpose Flexure Fixtures
In fact, the Image you show in your post is actually from their website: Deflectometer Plunger
Hope this answers your question.
These parts are purpose built and you would be very unlikely to find these other than as replacement parts for an item like this one (and likely very expensive for what they are).
If you were going to build such an instrument, you would want access to a machine shop (or great patience with hand tools) and the metalworking skills to go with it.
That said, most of these parts look pretty simple, at least the ones I can make out from the picture, and most of them could be fashioned from readily available bar stock and assembled with standard hardware.
You might consider adapting the design to use commonly-available bar stock, like aluminum extrusions, or perhaps fabricating some of the components from individual pieces of bar stock and fastening them together with screws (like a separate base and upright composing the circled part). Then the parts could be perhaps be fashioned using only a saw and a file (and patience :).