It's really very easy to make and won't break the bank even if you have to buy some new castors (available from any DIY shop). Do use a good, sturdy cardboard box though - this one has been sat in, dragged about sideways, turned upside-down and sat on... You get the picture. I got mine from a pile outside my local supermarket.
You will need:
A very sturdy cardboard box
coloured paper (red, blue, orange) and white paper
black marker pen
1 metre of sturdy rope
4 castors, ideally recycled
clear sticky tape
strong tape such as parcel or gaffer tape, in any colour
scissors or a paper knife
1. Using the knife or one blade of the scissors, make four small holes in the bottom of the box, where you want the wheels to be. The circumference of the holes should be smaller than than of your castors to ensure a snug fit.
2. Push the castors through the holes carefully so as not to rip the cardboard, then secure with lots of strong tape both on the outside and inside of the box. Make sure the wheels can still turn.
6. Cut two rectangles of blue paper for the front and back window and attach.
7. Cut four small squares of orange paper for the head- and taillights, attach two on the front and two on the back.
8. With a black marker pen and a ruler, draw around the windows to make them more realistic-looking.
10. Cut one large and one small rectangle out of the white paper, and give your bus a number and destination.