There are many resources for CAD blocks available on the 'net. I was able to find a few "dxf" files scattered about, but more often than not what I found were "dwg" files that could not be used with LibreCAD. I started drawing a few of my own and ended up going down that rabbit hole. The result was a reasonable collection that others might find useful so I created a GitHub repository specifically for dxf CAD blocks and posted what I have: dxfBlocks.
To download the collection, follow the link above, click the Clone or Download button and then "Download ZIP" to get the entire library. Refer to the LibreCAD User Manual - Blocks for details on using installing and using blocks.