Re: Need Boulders
Be aware that you may be liable for any damage to his vehicle by anything you place along the property line.
Having said that, any landscape supply should have the rocks you are looking for, and I would recommend 300 to 500 pounders, less likely the horses arse will be able to roll them into your yard out of "his" parking space. Another idea would be to plant a holly hedge or some other equally spiny plant that will not only attack him when he gets out of his car, but scratch the car up as well. We had some turd neighbors such as yours and the prickly bushes is the route we took.
Another good cure would be to start peeing on his car - preferably into it if he leaves the windows open, zip tie some washers to his driveline, stuff a potato down his exhaust pipe, siphon his gas, etc.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!