Does anyone know if there is a program to search for files in LANs?
Or is this possbile with nautilus? I already tried it with nautilus but there were no search results (no matter what I looked for).
When you refer to LANs you are probably talking about Windows networked computers and shares. In Linux you can mount those shares and you are able to browse the content of those shares with nautilus after mounting them.
After mounting a share on a mount point in the file system, nautilus will be able to find files inside those shares.
If you need instructions on how to mount a share in your Ubuntu computer have a look here.