There are four readily available methods to transfer a file to a Macintosh Classic. It is likely that all of them require you to purchase something.
1. LocalTalk networking - this is the old-fashioned serial port based networking that was the standard on classic Macs. Unfortunately, this doesn't work in any version of OS X. So you would need a more modern Mac, but still old enough to run OS 9 and have a serial port. This would be something like a beige Power Mac that has both Ethernet and serial.
2. Ethernet - as the Macintosh Classic has no Ethernet, and no internal expansion slot, you would need a SCSI Ethernet adapter. You would also need to make sure it came with a driver disk, because if you don't have that, you can't get the drivers on to the system! That would allow you to use AppleTalk-over-Ethernet, which works just fine between old and new Macs.
3. Floppy disk - any USB floppy drive will work on a modern Mac just fine, and the Classic can read 1.4 MB disks just fine.
4. Direct serial connection - using a terminal application to transfer files via serial. Would require a serial cable for the Classic, a USB-to-serial adapter for the modern computer, a null modem adapter, and likely a plug adapter to get the old-Macintosh-8-pin serial into a PC-standard 9-pin. You would then need some form of terminal software on the Classic. (Note: I have used this method to have my original Macintosh act as a serial terminal for my UNIX machine.)
