I thought about getting a HDMI splitter, but HDMI splitter will just split the connection which I don't want. I don't want to duplicate displays I want to extend them. So I guess adding a GPU is the safest option.Almost any video card will work here, so get whatever the cheapest one you can find. Though I would suggest something newer than older if possible.
If you only need one more monitor and you just need it to show something relatively unchanging, I'd also say a USB video adapter can work here as well.