System 7.1 is not available for download from Apple, but you can find complete 6.0.3, 6.0.5, 6.0.8, 7.0, 7.0.1 and 7.5.3 versions.
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/System/Older_System/System_6.0.x/
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/System/Older_System/System_7.0.x/
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -British/Macintosh/System/System_7.0/British__800K_/
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -British/Macintosh/System/System_7.0.1/
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/System/Older_System/System_7.5_Version_7.5.3/
http://support.apple.com/kb/TA33972
http://support.apple.com/kb/TA47341
Please note that the Macintosh SE is limited to handling 800K (2DD/DSDD) diskettes, unless it is an FDHD version. It is possible to install systems not on 800K floppies through other methods (network, external hard drive, et cetera).
Floppy disks (not 7.5.3) are supposed to be created using the Disk Copy 4.2 utility (the Make A Copy button).
http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English -North_American/Macintosh/Utilities/Disk_Copy/
System 7.5.3 (slow on a computer of this kind) must be installed in a different way (see the text file).
Jan