The workbench disk would normally come with the Amiga. Ask the person you bought it from if he/she has neglected to pass it on to you. Most Amiga users collect hundreds of floppies, and they do tend to get mixed up somewhat. O/w, you may still be able to pick one up second hand.
I only know of 3 ways to transfer floppies from PC to Amiga, each of which is probably not readily available to you. In this case, transfering the .rom file wouldn't help you anyway, so this is more for future reference.
1. Use a program called CrossDos on your Amiga, for which I believe you must have workbench running already. I believe it was available in Amiga OS, versions 2.? and higher. Haven't actually seen this working on a 500, but I guess it would be OK. It allows the Amiga to read PC floppy disks (usually 720kB type). I believe you can get this as a standalone program, but I don't know if it would work with rom v1.3, or how much RAM is needed etc. As CrossDos runs under workbench, you can't use it to load games or other programs which actually boot the Amiga.
2. Use a hardware hack/adapter to put a PC floppy drive onto an Amiga. Haven't tried this, and don't know if there are any issues with using such a drive to load programs without workbench already running.
3. Using special hardware called a Catweasel, installed on one's PC to allow the connection of an Amiga floppy drive to the PC. This is a bit costly, if I recall. This way, one can actually make Amiga floppy disks using your PC.
You may be best asking for someone else's help, who has an Amiga online, or workbench settup if you need to transfer files.
Don't forget, you can still use WinUAE to play games.
Have fun.