Can anyone tell me how I can get hold of one of those splendid looking ties?

Do you mean those ties with the broad green stripes alternating with "house" colours (carried also by caps, scarves and lapel badges in our early days)?
Or, had you in mind Joe's (uniform) School Uniform, an innovation presumably calculated to stamp his mark on the place?

Years at KBGS e.g. 1958-1964 (optional) 58 -65

This prompted me to look at my collection. Is the tie you require Dark blue base with two diagonal stripes, one yellow about an eighth of an inch and the other dark green quarter of an inch with a one and a half inch space between the paired stripes. In the space a row of two or three yellow three turreted castles about three tenths of an inch in height ?Ihave two of these but have no idea where you get them now.Fat lot of good that is to you Marcus.

Years at KBGS e.g. 1958-1964 (optional) 43-46

Current location (optional) Sassafras, Tasmania

Sorry, John not Marcus

Years at KBGS e.g. 1958-1964 (optional) 43-46

Current location (optional) Sassafras, Tasmania

The tie in question is shown in the photo section of the web page. Not the old house coloured ties as suggested by Brian

Years at KBGS e.g. 1958-1964 (optional) 58-61

Current location (optional) Blue Mountains, Australia