Where do I get the blueprint of the sewer lines on my property? Butte, Montana

Call the city plumbing department. Most municipalities require a drawing of the connecting private sewer lines.

If you can edit your post remove your street address.

City Hall. Hopefully. Not all localities have them on file. Sometimes you have to talk to the Maintenance Dept.
You are on city sewer, and not your own septic? You can sometimes tell a lot just by looking at the spot where your waste pipe leaves your basement.

Don't be surprised if City does not have blueprint of sewer lines on YOUR property on file. Where I live, City keeps track of their own sewer pipes as well as laterals that point towards each lot. They give you approximate location of the lateral from the nearest manhole, its size and depth. Once it crosses property line, you are on your own. City doesn't care. Every plumber does sewer installation his way and with no blueprints. If that is the case, you will have to call a Pipe Locating Service who will locate it for you

Milo