Some counties in Colorado have them available online through the county assessor's office. That's usually the larger counties that have money to spend on such things. The rural counties tend not to have much, if anything, online.

Olingers has the best idea. I too live in Ohio and my job requires constant property locates. Almost all counties auditors websites will have maps available some do not have aerial overlays though. Start by searhcing auditors website if they don't have what your looking for go to their office they will have it on file, but you need to know some basic info such as owner, address, or parcel ID. Another option is checking county website for GIS, many counties have them and many don't. Check engineers page, and home page they keep the links in all kinds of different places so it can be a chore to find.