I am working with LTspice and I need to put a BNC at the input of my amplifier. Where can I find this symbol?
LTspice is not a general-purpose schematic tool. There’s no point to such a symbol, since you must connect the input from a source. There are no connectors involved.
If you want to model a connector’s parasitics, you can design your own sub circuit for it, and can then also provide a custom symbol for it.
Do not use LTspice as a schematic tool. You’ll be fighting an uphill battle that’s not worth it.