What brand of machine is it?
There are a number of vintage sewing machine groups for specific brands on Yahoo! (
http://groups.yahoo.com ). If you join the group for your machine, often there are people there who specialize in that brand and either have parts or can direct you to others likely to have parts for it. There's also a "wefixit" or "wecanfixit" group for sewing machines on Yahoo!; that would be another good group to join. Just do a search on groups for your brand of machine and on vintage sewing machines.