If you're taking the photo yourself, it's possible to get the best photos for reference.
You want to get as straight-on and level as you can, i.e. minimising the angle you're looking at the bus with. As soon as you start to look at something from an angle it tapers as it gets further away. You can achieve a more-level photo of something taller than you by finding higher ground or just moving further away.
You want to minimise lens distortion. I think that point is usually around the mid-point of optical zoom, but it may vary depending on your lens. Again, standing a bit further away can help. If you've got lens distortion you'll find that thing that should be straight are curved, more so nearer the edges of the image.
For reference, I took
this photo from perhaps 15 metres away from the bus directly in front of it. Notice how the sides are nice and vertical. There'll still be some small distortion in it, but that can be fixed in photo-editing software fairly easily without losing quality.
Remember that your model doesn't have to be perfect, it just has to look close enough to the real thing that nobody notices it's wrong.