+1 for music stores for boxes.
If youre posting, make sure the power cable is secured in the amp firmly. If it moves around it could hit stuff and damage things like your speaker etc. Just put some bubble wraps in or you could tape it down.
If youre meticulous/sad with too much time like me, you could do this and make corner supports:
- cut a thick cardboard into squares say, about 30cmx30cm
- fold it into 4 then open again
- cut along one of the four lines to the centre of the square
- fold it at right angles and tape the overlapping bits
- make 7 more!

:lol: