Some cities or counties have them online, but most don't. What I do if a client doesn't have their plat is this:
- I measure whatever portion of the house is relevant to the area I'm designing, but at least one whole side
- I locate a few key detail points (driveway, trees, whatever)
- I then get the aerial imagery from GoiLawn and print it out to scale and use that for a base just like I would a plat.
It's not as accurate as a plat, but if I also have some exact field measurements of key areas, it's close enough for a planting plan.
If you're doing hardscapes or carpentry stuff and you can't get a plat, you need to suck it up and do a really thorough job measuring. It's a hassle, but it beats screwing up an estimate.