What operating system are you looking for?
http://www.lowendmac.com/ppc/power-macintosh-5400.html
This model can use OS 7.5.1 to 9.x.
Apple has 7.5.3 available on their website
http://www.info.apple.com/support/oldersoftwarelist.html
Look down to where you see:
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_01of19.smi.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_02of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_03of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_04of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_05of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_06of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_07of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_08of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_09of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_10of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_11of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_12of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_13of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_14of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_15of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_16of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_17of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_18of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_19of19.part.bin
Macintosh/System/Older_System/System_7.5_Version_7.5.3/System_7.5.3_Info.txt
Note:
Instructions:
This software is available as 19 parts of a self-mounting Disk Copy
image. Download all 19 parts to your hard drive and then double-click
on the first part to mount the compressed disk image on your desktop.
Note that self-mounting Disk Images require Mac OS version 7.0.1 or
later. If you are using a version of Mac OS prior to this, you can
download the System 7.5 Network Access floppy disk and boot your
Macintosh from that to use this software.
So you can download the images to your current HD, unarchive them to get an image, then copy it to something bootable to reformat your main drive and install the OS.
Any other OS is not free and will have to be purchased legaly or found someway else.