If you can get it scanned -- probably need about 1200 or 2400 dpi for an enlargement of that size -- I get my oversize prints made at Adoramapix.com.
If you can't find a way to get it scanned, I would ask some local photographers, especially if you can find any still in business that pre-date digital photography. A museum or historical society that does image preservation may also be able to help you get it scanned.
There are also ways to use a digital camera to photograph the negative, which would be akin to scanning it, but the sharpness would drop some and you might not be able to get the dpi you need that way unless you're using a very high-end professional model. You can Google it and find a number of methods, including attachments that fit in front of a lens to hold the negative. I have one of these and it wasn't expensive.
You might also try searching at Google for something like "oversize prints from 35mm negatives" and maybe a company that does this will show up.
Hope this helps.