We have a couple of well respected websites over here in north america where you can buy ultrasound machines, shipper, nitro tanks etc in good used condition. There are also websites selling new equipment but prices are significantly higher. Also a number of my breeder friends over here sell on FB AI dedicated sales group pages. I don't know if you have the same in the UK but maybe worth an intense google search, or perhaps some of the UK breeders know the name of them.