check out your municipality's web site. Often the tax maps are on line. Your site plan though usually is more similar to the survey as it shows location of structures, pools, fence etc. You can take a copy of the survey and use this to create a plot plan if you are planning on an addition or other improvements. I have found plot plans to be less exact and not stamped vs surveys. You could also take tax maps and sketch in your improvements but this will require you to take measurements etc. When in doubt, get a survey. My neighbor built a large patio for his hot tub on neighbor's property and had to rip it out.

Anet:

A plot plan is usually drawn up by the original builder and, after the original owner sells, often lost. If you just want the Tax map with dimensions, any Realtor can get it on line. Otherwise, you need a surveyor. If it is in connectoin with a sale, the buyer usually needs it for it for the mortgage and pays for it then. If you need an elaborate plot plan with grades, etc., it might cost a little more. A new survey is a very good thing for the buyer to have, even if not manditory. It clears up any changes that no one has noticed, such as encroachments and new easements.

If you're buying, the seller might have one. Otherwise, you can order a survey yourself for any property you own.