Where can I get reference pictures of old people?

I would do a google search of old people or senior citizens, then go to the images tab and you will get pictures galore. Link

That does answer the question somewhat, but what I really want are high-resolution pictures of old people in the nude.

There are a number of stock photo sites that do high-res photographs. Some free, some you would have to pay for.

http://www.istockphoto.com/
http://www.alamy.com/
http://www.corbisimages.com/
http://www.fotosearch.com/
http://www.bigstockphoto.com/

You need access to a medical library.

In case you don’t want to pay and want a large image, try wikimedia and look for elderly folk. Are you looking for nude folks? Try research studies [and add the face from somewhere else as it will not be shown]
Try morgue shots if lifelike poses are not an issue
Hire a model. Not all models are young. If you are in the DG area I can recommend someone.