Well, there exist rack consoles for datacenter use which can be found used on eBay. However, you might need a video converter since they are VGA only; rackmount servers only have VGA video outputs.
The least expensive option would be to buy a second small monitor.
anything with purely USB won't work in BIOS or recovery system etc where a matching driver for whatever USB-graphics thing it is using isn't loaded.
If your server has HDMI out, you can use a cheap HDMI-USB-capture dongle with a laptop, smartphone or tablet. (small portable HDMI screens are also available, but for occasional use buying one is overkill). VGA capture modules are more expensive, but should also exist.
Otherwise a traditional way is a serial console, which again would need a serial adapter for whatever device you use with it - again depends if your hardware has a serial port and (if BIOS is relevant) if the BIOS talks over it.