The typical method of dumping the ROMs is to desolder them (unless you're lucky enough to have them socketed) and use an EPROM programmer to read the data from them. There's a good how-to by The Guru (who's dumped many arcade ROMs for MAME) here:
http://unemulated.emuunlim.com/how_to_dump/index.html
If you don't have the tools, electronics skills, motor control or confidence to dump the ROMs, you may have to look for someone else to do it for you.
As for using a file you've found online somewhere: I am not a lawyer, so I can't say for certain whether owning the machine entitles you to do so. Nintendo certainly don't think so. You may want to read through the applicable laws in your country, or ask a lawyer.