I've used them in the past in NY. Unfortunately there is no standard way to get them. You just have to dig. In the two counties that I was interested in, I dug through their websites, usually in the tax assessors portion and found them there. At the time, I had to overlay them on google maps to create a GPX file which had the appropriate coordinates in it. Both counties used two different systems (3rd party providers for the maps), so it wasn't the same process to get the maps I needed.
One thing you may also want to try for your purposes is to check out the state level resources. Sometimes, the state DNR/fish&game/DEC sites will post fairly detailed maps with private prop boundaries.
A few months back Outdoor Life printed an article on GPS maps. I tore it out and it is on my desk in front of me right now. I haven't had a chance to check out the sites they noted, but. For pay maps, they identified huntinggpsmaps.com for free maps they called out GPSfiledepot.com. I haven't had a chance to check either out.
Hope this helps a little, let us know what you find out.