When it comes to geometry problems (such as the ones I’m faced with when building isometric perspective games), I typically do better if I can sketch out my desired outcome. Doing so can be a challenge in itself if my sketches aren’t proportionally correct, so I just sort of wing it.
Fed up with winging it, I set out to find some isometric graph paper to purchase, and instead found what I think is one of the coolest sites I’ve ever seen. Behold, the graph paper generator. Pick from several different styles of graph paper and tweak colors and grid size to your hearts content, generating a PDF that you can print when finished.