They were widely available in the , in those days, 'catalogues' for home shopping, where you could by kettles, shirts, teas-maids, mirrors and all sorts of tat. Mine came from a Freeman's catalogue, cost around £40 and was paid-up at ten shillings a month or something like that (50p!)
I added the worst 'bass amplifier' I have ever seen - a Hawk 15 - and immediately became a rock-god at school.
Here it is in action almost 50 years ago.